Entry point assertion class for custom assertions

Currently, `assertThat` methods from custom asserts(`SpanAssert`,
`SpansAssert`, `TracerAssert`) do not have an entry point assertion
class. Hence, users need to static import individual assert classes.

This change introduces an entry point assertion class. Users now simply
static import this class and all `assertThat` methods are available for
use.

micrometer-tracing-tests/micrometer-tracing-test/src/main/java/io/micrometer/tracing/test/simple/TracingAssertions.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2022 VMware, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micrometer.tracing.test.simple;
+
+import java.util.Collection;
+
+import io.micrometer.tracing.exporter.FinishedSpan;
+
+/**
+ * Entry point assertions.
+ *
+ * @author Tadaya Tsuyukubo
+ * @since 1.0.0
+ */
+public class TracingAssertions {
+
+    @SuppressWarnings("rawtypes")
+    public static SpanAssert assertThat(FinishedSpan actual) {
+        return new SpanAssert(actual);
+    }
+
+    public static SpansAssert assertThat(Collection<FinishedSpan> actual) {
+        return new SpansAssert(actual);
+    }
+
+    public static TracerAssert assertThat(SimpleTracer actual) {
+        return new TracerAssert(actual);
+    }
+
+}