Fix prometheusexporter droping the OTEL resource labels

[krak3n]

Description:
Added support for resource_to_telemetry_conversion to the Promethus Exporter so resource attributes are added as labels to metrics.

Link to tracking Issue: Fixes #2498

Testing:
Added new test cases to ensure resource labels are included in the prometheus metrics output when resource_to_telemetry_conversion is enabled.

Two existing test cases were amended due to their requirement via a type assertion to have access to the underlying prometheusExporter which is no longer the case since this was changed to a *exporterhelper.metricsExporter. To accommodate this a wrapper was added which allowed the type assertion to work and the test cases to pass.

Manual testing was done by building and running the otelcol locally with the following configuration:

receivers:
  otlp:
    protocols:
      grpc:

exporters:
  prometheus:
    endpoint: "0.0.0.0:8889"
    resource_to_telemetry_conversion:
      enabled: true

processors:
  batch:

service:
  pipelines:
    metrics:
      receivers: [otlp]
      processors: [batch]
      exporters: [prometheus]

A service was written to send metrics with a resource to the locally running otelcol and the prometheus metrics endpoint was queried and observed that metrics now had resource labels attached to them, e.g:

# HELP example_bar
# TYPE example_bar counter
example_bar{deployment_environment="development",foo="bar",service_name="foo-srv",service_version="v1.2.3",telemetry_sdk_language="go",telemetry_sdk_name="opentelemetry",telemetry_sdk_version="0.19.0"} 193

Documentation:

The Prometheus Exporter README.md was updated to include documentation about the new resource_to_telemetry_conversion configuration option.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Chris Reeves (0be56e9, 3286b18, db5077d)

[codecov]

Codecov Report

Merging #2899 (1d5a944) into main (13e4566) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2899   +/-   ##
=======================================
  Coverage   91.63%   91.64%           
=======================================
  Files         312      312           
  Lines       15404    15420   +16     
=======================================
+ Hits        14116    14132   +16     
  Misses        881      881           
  Partials      407      407           

Impacted Files	Coverage Δ
exporter/prometheusexporter/config.go	100.00% <ø> (ø)
exporter/prometheusexporter/factory.go	100.00% <100.00%> (ø)

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[langecode]

I was looking into this exact same feature with the prometheusremotewriteexporter. Was just wondering - strictly speaking it is "resource attributes as labels"? I guess Prometheus uses the term "label" instead of "tag"?

[krak3n]

As @bogdandrutu I'll switch it to a convention thats used over in the contrib repo so things are kept consistent, so this will change too

        resource_to_telemetry_conversion:
            enabled: true

[bogdandrutu]

seems to be tags not tag

[bogdandrutu]

In contrib where we use this, we have config like:

	exporterhelper.ResourceToTelemetrySettings `mapstructure:"resource_to_telemetry_conversion"`

so the config will be:

   resource_to_telemetry_conversion:
      enabled: true

If you don't like that name maybe embed that settings but use the name resource_attributes_as_tags? What do you think?

[krak3n]

ah cool, it should be consistent with existing implementations so happy to switch it up to:

        resource_to_telemetry_conversion:
            enabled: true

[krak3n]

@bogdandrutu I've pushed the required changes, sorry for the delay 👍