Add helper functions/conventions/documentation to help contrib components add support to custom telemetry

[rapphil]

Is your feature request related to a problem? Please describe.

Some components in the contrib repository need to add additional telemetry besides the one provided by the {exporter,processor}helper packages.

I'd like that the collector core provided helper functions that simplify process of adding custom telemetry in contrib components. One good example is the BuildCustomMetricName function in the processorhelper package. There is no equivalent of this function for the exporterhelper package.

Describe the solution you'd like
Helper functions and documentation about how to add custom telemetry to contrib modules.

Optionally add helper functions/framework for testing custom telemetry, similar to what we have in this package:

opentelemetry-collector/obsreport/obsreporttest/obsreporttest.go

Line 136 in 9a03cd3

func SetupTelemetry(id component.ID) (TestTelemetry, error) {

Additional context
open-telemetry/opentelemetry-collector-contrib#29729