[obsreport] Replace Prometheus namespace with metrics prefix

[dmitryax]

"otelcol" part of the exposed metrics that is set as Prometheus namespace is replaced the same explicit metrics prefix. This change brings names consistency between metrics that can be reported with enabled or disabled "telemetry.useOtelForInternalMetrics" feature gate.

Resolves #5882

[codecov]

Codecov Report

Merging #5939 (0bb640b) into main (f0d69d6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5939      +/-   ##
==========================================
- Coverage   91.93%   91.93%   -0.01%     
==========================================
  Files         200      200              
  Lines       12406    12404       -2     
==========================================
- Hits        11406    11404       -2     
  Misses        789      789              
  Partials      211      211              

Impacted Files	Coverage Δ
exporter/exporterhelper/obsreport.go	100.00% <100.00%> (ø)
obsreport/obsreport_exporter.go	100.00% <100.00%> (ø)
obsreport/obsreport_receiver.go	100.00% <100.00%> (ø)
obsreport/obsreport_scraper.go	100.00% <100.00%> (ø)
obsreport/obsreporttest/obsreporttest.go	95.62% <100.00%> (ø)
service/internal/telemetry/process_telemetry.go	71.55% <100.00%> (ø)
service/telemetry.go	90.25% <100.00%> (-0.13%)	⬇️

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[codeboten]

Just one comment, otherwise this looks good. Hurray for consistency :)

[codeboten]

To be clear to most users this will not be a breaking change as the prometheus metric exposed will be the same.

Suggested change

	- "otelcol" part of the exposed metrics that is set as Prometheus namespace is replaced the same explicit metrics prefix (#5939)
	- Update `otelcol_` part of the exposed Prometheus metrics to be set via a metric prefix instead of a namespace (#5939)

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[paivagustavo]

This can also be accomplished by wrapping the prometheus.Registry before registering the otel-go prom exporter's Collector, which would not break any additional metric:
https://github.com/open-telemetry/opentelemetry-collector/pull/6222/files#diff-52da5566fa270d6a253f7a301b66f4a1dbda7c2c42f3781f8357efe7f08dff10R255

edit:
see #6223

[bogdandrutu]

#6223 closes this