Add OTel resource attribute promotion proposal

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

proposals/2024-07-15-otel-resource-attribute-promotion.md

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+# OTel resource attribute promotion
+
+* **Owners:**
+  * Arve Knudsen [@aknuds1](https://github.com/aknuds1) [arve.knudsen@grafana.com](mailto:arve.knudsen@grafana.com)
+
+* **Implementation Status:** Partially implemented
+
+* **Related Issues and PRs:**
+  * [WIP: OTLP Translator prometheusremotewrite: Support resource attribute promotion](https://github.com/prometheus/prometheus/pull/14200)
+
+* **Other docs or links:**
+
+> This proposal collects the requirements and implementation proposals for supporting OTel resource attribute promotion to labels.
+
+## Why
+
+Currently, Prometheus encodes OpenTelemetry (OTel for short) [resource attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md) as labels of the `target_info` metric.
+OTel resource attributes model metadata about the environment producing metrics received by the backend (e.g. Prometheus).
+Typically, OTel users want to include some of these attributes (as `target_info` labels) in their Prometheus query results, to correlate them with entities of theirs (e.g. K8s pods).
+
+Based on user demand, it would be preferable if Prometheus were to have better UX for including OTel resource attributes in query results.
+The current solution is to join with `target_info in queries, to pick also the labels one is interested in (corresponding to OTel resource attributes).
+This requires relatively advanced knowledge of PromQL though and is a barrier to many users.
+Take as an example querying HTTP request rates per K8s cluster and status code, while having to join with the `target_info` metric to obtain the `k8s.cluster.name` resource attribute (encoded as `k8s_cluster_name`):
+
+```promql
+# Join with target_info on job and instance labels, to include k8s_cluster_name.
+sum by (k8s_cluster_name, http_status_code) (
+    rate(http_server_request_duration_seconds_count[2m])
+  * on (job, instance) group_left (k8s_cluster_name)
+    target_info
+)
+```
+
+### Pitfalls of the current solution
+
+As already mentioned, the current solution of including OTel resource attributes in query results through join queries represents a technical barrier to users.
+Also, it requires the user to know which `target_info` labels can be joined on (i.e., `job` and `instance`), plus which labels represent the various OTel resource attributes.
+All in all, the UX for including OTel resource attributes in Prometheus query results is not very smooth.
+
+## Goals
+
+Goals and use cases for the solution as proposed in [How](#how):
+
+* Support, in the OTLP endpoint, automatic promotion of a configurable set of OTel resource attributes to metric labels.
+
+### Audience
+
+Prometheus maintainers.
+
+## How
+
+* Make the OTLP endpoint support a configurable set of OTel resource attributes to promote to metric labels.
+* Add a Prometheus configuration parameter for which OTel resource attributes to promote.
+
+With OTel resource attribute promotion configured to `[k8s.cluster.name]`, we can simplify the previously given PromQL join example as follows:
+
+```
+sum by (k8s_cluster_name, http_status_code) (
+  rate(http_server_request_duration_seconds_count[2m])
+)
+```
+
+## Alternatives
+
+### Simplify joins with info metrics in PromQL
+
+Instead of promoting selected OTel resource attributes to labels at ingest time, another [proposal](https://github.com/prometheus/proposals/pull/37) is to simplify the joining with `target_info` in queries.
+These proposals are not necessarily competing though, as the respective proposed features can co-exist.
+
+#### Pros
+
+* Avoids having to add more labels to metrics than strictly required to identify them.
+* Avoids series churn when one or more of the promoted OTel resource attributes change.
+* More labels per metric increases CPU/memory usage.
+* Avoids the user having to decide up front which OTel resource attributes to promote at ingestion time.
+* Avoids series churn when the user changes which OTel resource attributes to promote.
+* Simply improves the UX for the existing solution of encoding OTel resource attributes as `target_info` labels.
+
+#### Cons
+
+* Much more complicated to implement.
+* Requires the user to call `info` in their queries.
+
+## Action Plan
+
+The tasks to do in order to migrate to the new idea.
+
+* [ ] https://github.com/prometheus/prometheus/pull/14200