Metrics-generator should take sampling rate into account

[kvrhdn]

Is your feature request related to a problem? Please describe.
Traces are often downsampled before they are ingested by Tempo. Since the metrics-generator emits metrics based upon ingest volume, these metrics will also be downsampled.

For example, traces_spanmetrics_calls_total emits how often a specific operation happend. If traces are downsampled by constant sampling percentage 50%, the metric will be 50% lower as well than reality. This is likely misleading as you have to highly aware of downsampling happening in the trace pipeline.

With more involved down sampling setups (tail sampling, per operation sampling) it becomes impossible to calculate the actual value.

Describe the solution you'd like
The metrics-generator should pick up the sampling rate used and adjust the emitted metrics. Ideally, it extracts the sampling rate from metadata attached to ingested spans.

Describe alternatives you've considered

Additional context

[github-actions]

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