Add metrics to understand cost of telemetry

[pavolloffay]

Component(s)

No response

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

I would like to use the collector to understand the cost of telemetry data collection. Usually, observability vendors charge per ingest volume.

Describe the solution you'd like

I would like to have internal telemetry metrics that will help me understand how much data is collector processing/sending.

I want to understand how much each instrumented process (service) reports bytes. What is average span size and attribute cardinality?

Describe alternatives you've considered

No response

Additional context

No response

[crobert-1]

Am I correct in understanding that you're proposing to add these to the internal metrics the collector exposes? Would these values be per telemetry pipeline, or for the running collector as a whole?

[crobert-1]

Related: open-telemetry/oteps#249

[github-actions]

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