Allow Prometheus exporter to add resource attributes to metric attrib…

…utes (#3761)

Fixes #3705 

## Changes

Allowing exporters (e.g. Prometheus exporter) to add the resource
attributes to each exported metric attributes

CHANGELOG.md

@@ -16,6 +16,9 @@ release.
 
 ### Metrics
 
+- Add optional configuration for Prometheus exporters to promote resource attributes to metric attributes
+  ([#3761](https://github.com/open-telemetry/opentelemetry-specification/pull/3761))
+
 ### Logs
 
 - Add definition for standard output log record exporter.

specification/compatibility/prometheus_and_openmetrics.md

@@ -385,11 +385,11 @@ OpenMetrics exemplar unless they would exceed the OpenMetrics
 
 In SDK Prometheus (pull) exporters, resource attributes SHOULD be converted to
 a single [`target_info` metric](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#supporting-target-metadata-in-both-push-based-and-pull-based-systems)
-if the resource is not [empty](../resource/sdk.md#the-empty-resource);
-otherwise, they MUST be dropped, and MUST NOT be attached as labels to other
-metric families. The target_info metric MUST be an info-typed metric whose
-labels MUST include the resource attributes, and MUST NOT include any other
-labels. There MUST be at most one target_info metric exposed on an SDK
+if the resource is not [empty](../resource/sdk.md#the-empty-resource).
+The resource attributes MAY be copied to labels of exported metric families
+if required by the exporter configuration, or MUST be dropped. The target_info metric MUST be an info-typed
+metric whose labels MUST include the resource attributes, and MUST NOT include
+any other labels. There MUST be at most one target_info metric exposed on an SDK
 Prometheus endpoint.
 
 In the Collector's Prometheus pull and push (remote-write) exporters, it is

specification/metrics/sdk_exporters/prometheus.md

@@ -25,3 +25,9 @@ A Prometheus Exporter MAY support [OpenMetrics Text
 Format](https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#openmetrics-text-format),
 including the
 [Exemplars](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars).
+
+A Prometheus Exporter MAY offer configuration to add resource attributes as metric attributes.
+By default, it MUST NOT add any resource attributes as metric attributes.
+The configuration SHOULD allow the user to select which resource attributes to copy (e.g.
+include / exclude or regular expression based). Copied Resource attributes MUST NOT be
+excluded from target_info.