Include resource attributes as target_info for Prometheus exporters

[robertcoltheart]

Fixes #3087

Changes

Adds a target_info section to the Prometheus output.

target_info is only displayed for clients using the Accept: application/openmetrics-text header, since Prometheus will break on reading info types with servers that don't send the Content-Type: application/openmetrics-text header.

Example output that shows target_info with resource attributes:

# TYPE target info
# HELP target Target metadata
target_info{deployment_environment="Production",service_name="otel-test",service_version="1.0.0.0",service_instance_id="GLIP",telemetry_sdk_name="opentelemetry",telemetry_sdk_language="dotnet",telemetry_sdk_version="0.0.0-alpha.0.2851"} 1
# TYPE otel_scope_info info
# HELP otel_scope_info Scope metadata
otel_scope_info{otel_scope_name="Microsoft.AspNetCore.Server.Kestrel"} 1
# TYPE kestrel_active_connections gauge
# HELP kestrel_active_connections Number of connections that are currently active on the server.
kestrel_active_connections{otel_scope_name="Microsoft.AspNetCore.Server.Kestrel",network_transport="tcp",network_type="ipv6",server_address="::1",server_port="5000"} 1 1709377047.805
kestrel_active_connections{otel_scope_name="Microsoft.AspNetCore.Server.Kestrel",network_transport="tcp",network_type="ipv4",server_address="127.0.0.1",server_port="5000"} 1 1709377047.805
# EOF

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 85.57%. Comparing base (6250307) to head (317ce4c).
Report is 138 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5407      +/-   ##
==========================================
+ Coverage   83.38%   85.57%   +2.19%     
==========================================
  Files         297      289       -8     
  Lines       12531    12640     +109     
==========================================
+ Hits        10449    10817     +368     
+ Misses       2082     1823     -259     

Flag	Coverage Δ
unittests	?
unittests-Solution-Stable	84.96% <83.33%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...etheus.HttpListener/Internal/PrometheusExporter.cs	100.00% <100.00%> (ø)
...heus.HttpListener/Internal/PrometheusSerializer.cs	85.79% <94.11%> (+0.93%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	76.37% <66.66%> (-1.40%)	⬇️

... and 64 files with indirect coverage changes

[vishweshbankwar]

@robertcoltheart Could you split the PR in to two parts?

1. Add target_info metric.
2. Ability to add resource attribute to individual metrics.

It will help in reviewing the PR and separating public API addition from an internal change. Thanks!

[robertcoltheart]

Thanks @vishweshbankwar, updated per recommendations. Will raise the 2nd PR once this is merged.

[robertcoltheart]

I've added caching for target_info, this should be good to go now

[cijothomas]

telemetry_sdk_version="0.0.0-alpha.0.2851

how come! 🤣 we didn't have prometheus exporter that far !

[robertcoltheart]

telemetry_sdk_version="0.0.0-alpha.0.2851

how come! 🤣 we didn't have prometheus exporter that far !

Lol yeah I'm guessing this was because I was running it locally? Not sure where the version number is coming from.