Add documentation and sample

Signed-off-by: Russ Cam <russcam@canva.com>

guides/index_templates.md

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+- [Index templates](#index-templates)
+    - [Setup](#setup)
+
+
+# Index templates
+
+Index templates let you initialize new indexes with predefined mappings and settings.
+For example, if you continuously index log data, you can define an index template so that all of these indexes have
+the same number of shards and replicas.
+
+## Setup
+
+To get started, first create a client, create an index template. In this example, an index template is created,
+composed of two component templates:
+
+- `index-settings` which specifies index settings such as shard configuration and slowlog settings
+- `index-mappings` which specifies the field mappings
+
+```java
+import org.apache.hc.core5.http.HttpHost;
+
+final HttpHost[] hosts = new HttpHost[] {
+    new HttpHost("http", "localhost", 9200)
+  };
+
+final OpenSearchTransport transport = ApacheHttpClient5TransportBuilder
+    .builder(hosts)
+    .setMapper(new JacksonJsonpMapper())
+    .build();
+OpenSearchClient client = new OpenSearchClient(transport);
+
+final var indexSettingsComponentTemplate = "index-settings";
+PutComponentTemplateRequest putComponentTemplateRequest = PutComponentTemplateRequest.of(
+    c -> c.name(indexSettingsComponentTemplate)
+        .settings(
+            s -> s.numberOfShards("2")
+                .numberOfReplicas("1")
+                .indexing(
+                    i -> i.slowlog(
+                        sl -> sl.level("info")
+                            .reformat(true)
+                            .threshold(th -> th.index(ith -> ith.warn(Time.of(t -> t.time("2s")))))
+                    )
+                )
+                .search(
+                    se -> se.slowlog(sl -> sl.level("info").threshold(th -> th.query(q -> q.warn(Time.of(t -> t.time("2s"))))))
+                )
+        )
+);
+client.cluster().putComponentTemplate(putComponentTemplateRequest);
+
+final var indexMappingsComponentTemplate = "index-mappings";
+putComponentTemplateRequest = PutComponentTemplateRequest.of(
+    c -> c.name(indexMappingsComponentTemplate).mappings(m -> m.properties("age", p -> p.integer(i -> i)))
+);
+client.cluster().putComponentTemplate(putComponentTemplateRequest);
+
+final var indexTemplateName = "my-index-template";
+PutIndexTemplateRequest putIndexTemplateRequest = PutIndexTemplateRequest.of(
+    it -> it.name(indexTemplateName)
+        .indexPatterns("my-index-*")
+        .composedOf(List.of(indexSettingsComponentTemplate, indexMappingsComponentTemplate))
+);
+
+client.indices().putIndexTemplate(putIndexTemplateRequest);
+```
+
+## Usage
+
+Create an index with a name that matches the index template's `indexPatterns`
+
+```java
+String indexName = "my-index-1";
+client.indices().create(CreateIndexRequest.of(c -> c.index(indexName)));
+```
+
+The index will be created with the index settings and mappings defined by the `my-index-template` index template.
+
+You can find a working sample of the above code in [IndexTemplates.java](../samples/src/main/java/org/opensearch/client/samples/IndexTemplates.java).

samples/src/main/java/org/opensearch/client/samples/IndexTemplates.java

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+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.client.samples;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.opensearch.client.opensearch._types.Time;
+import org.opensearch.client.opensearch.cluster.PutComponentTemplateRequest;
+import org.opensearch.client.opensearch.indices.*;
+
+import java.util.List;
+
+/**
+ * Run with: <c>./gradlew :samples:run -Dsamples.mainClass=IndexTemplates</c>
+ */
+public class IndexTemplates {
+    private static final Logger LOGGER = LogManager.getLogger(IndexTemplates.class);
+
+    public static void main(String[] args) {
+        try {
+            var client = SampleClient.create();
+
+            var version = client.info().version();
+            LOGGER.info("Server: {}@{}", version.distribution(), version.number());
+
+            final var indexTemplateName = "my-index-template";
+
+            if (!client.indices().existsIndexTemplate(t -> t.name(indexTemplateName)).value()) {
+                DeleteIndexTemplateRequest deleteIndexTemplateRequest = DeleteIndexTemplateRequest.of(i -> i.name(indexTemplateName));
+
+                LOGGER.info("Deleting index template {}", indexTemplateName);
+                client.indices().deleteIndexTemplate(deleteIndexTemplateRequest);
+            }
+
+            // Create an index template composed of two component templates, one for index settings, and one for mappings
+            String indexSettingsComponentTemplate = "index-settings";
+            PutComponentTemplateRequest putComponentTemplateRequest = PutComponentTemplateRequest.of(
+                c -> c.name(indexSettingsComponentTemplate)
+                    .settings(
+                        s -> s.numberOfShards("2")
+                            .numberOfReplicas("1")
+                            .indexing(
+                                i -> i.slowlog(
+                                    sl -> sl.level("info")
+                                        .reformat(true)
+                                        .threshold(th -> th.index(ith -> ith.warn(Time.of(t -> t.time("2s")))))
+                                )
+                            )
+                            .search(
+                                se -> se.slowlog(sl -> sl.level("info").threshold(th -> th.query(q -> q.warn(Time.of(t -> t.time("2s"))))))
+                            )
+                    )
+            );
+            LOGGER.info("Creating component template {}", indexSettingsComponentTemplate);
+            client.cluster().putComponentTemplate(putComponentTemplateRequest);
+
+            String indexMappingsComponentTemplate = "index-mappings";
+            putComponentTemplateRequest = PutComponentTemplateRequest.of(
+                c -> c.name(indexMappingsComponentTemplate).mappings(m -> m.properties("age", p -> p.integer(i -> i)))
+            );
+            LOGGER.info("Creating component template {}", indexMappingsComponentTemplate);
+            client.cluster().putComponentTemplate(putComponentTemplateRequest);
+
+            PutIndexTemplateRequest putIndexTemplateRequest = PutIndexTemplateRequest.of(
+                it -> it.name(indexTemplateName)
+                    .indexPatterns("my-index-*")
+                    .composedOf(List.of(indexSettingsComponentTemplate, indexMappingsComponentTemplate))
+            );
+
+            LOGGER.info("Creating index template {}", indexTemplateName);
+            client.indices().putIndexTemplate(putIndexTemplateRequest);
+
+            String indexName = "my-index-1";
+            if (client.indices().exists(r -> r.index(indexName)).value()) {
+                LOGGER.info("Deleting index {}", indexName);
+                client.indices().delete(DeleteIndexRequest.of(d -> d.index(indexName)));
+            }
+
+            LOGGER.info("Creating index {}", indexName);
+            client.indices().create(CreateIndexRequest.of(c -> c.index(indexName)));
+
+            GetMappingResponse getMappingResponse = client.indices().getMapping(GetMappingRequest.of(m -> m.index(indexName)));
+            // mappings for the index should contain those defined in component template
+            LOGGER.info("Mappings {} found for index {}", getMappingResponse.result().get(indexName).mappings(), indexName);
+
+            GetIndicesSettingsResponse getSettingsResponse = client.indices()
+                .getSettings(GetIndicesSettingsRequest.of(m -> m.index(indexName)));
+            // settings for the index should contain those defined in component template
+            LOGGER.info("Settings {} found for index {}", getSettingsResponse.result().get(indexName).settings(), indexName);
+
+            LOGGER.info("Deleting index {}", indexName);
+            client.indices().delete(DeleteIndexRequest.of(d -> d.index(indexName)));
+        } catch (Exception e) {
+            LOGGER.error("Unexpected exception", e);
+        }
+    }
+}