prometheus: add WithNamespace option to prefix metrics

[paivagustavo]

Adds a WithNamespace option to the prometheus exporter.

Currently, the lack of such option blocks the otel collector: open-telemetry/opentelemetry-collector#7454

To finalize the collector migration to otel-go and start enabling it by default, both instrumentations must be compatible.

Collector's OpenCensus instrumentation prefixes metrics with the otelcol_ prefix.
To match that behavior in the otel-go instrumentation, we tried to wrap a prometheus.Registry with the otelcol_ prefix.

But since Otel Go resource attributes are exported as part of the target_info metric, which is also prefixed and becomes otelcol_target_info, for example:

# HELP otelcol_otel_scope_info Instrumentation Scope metadata
# TYPE otelcol_otel_scope_info gauge
otelcol_otel_scope_info{otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 1
# HELP otelcol_target_info Target metadata
# TYPE otelcol_target_info gauge
otelcol_target_info{service_name="prometheus_test",telemetry_sdk_language="go",telemetry_sdk_name="opentelemetry",telemetry_sdk_version="latest"} 1

This make it impossible for the OpenTelemetry Collector's Prometheus Receiver to properly parse the target_info and otel_scope_info into the appropriated resource attributes and scope attributes. For that reason, the Otel Go instrumentation is not compatible with the existing OpenCensus instrumentation.

Adding such a common option in the prometheus instrumentation would unblock the Collector's migration to Otel Go and also other external users such as: #3405 (comment)

For reference, this option is also available on collector's prometheus exporter

closes #3437

[codecov]

Codecov Report

Merging #3970 (5f89a61) into main (02fa1e2) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

❗ Current head 5f89a61 differs from pull request most recent head 0976db2. Consider uploading reports for the commit 0976db2 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3970     +/-   ##
=======================================
- Coverage   82.0%   82.0%   -0.1%     
=======================================
  Files        171     171             
  Lines      12955   12965     +10     
=======================================
+ Hits       10635   10641      +6     
- Misses      2100    2104      +4     
  Partials     220     220             

Impacted Files	Coverage Δ
exporters/prometheus/config.go	100.0% <100.0%> (ø)
exporters/prometheus/exporter.go	83.9% <100.0%> (+0.2%)	⬆️

... and 3 files with indirect coverage changes