Add histogram to track number samples/metadata/span sent per request

[arajkumar]

This commit also removes the metric promscale_ingest_items_received_total which is not used anywhere and the same information can be now obtained using the newly introduced promscale_ingest_items_received_* histogram metric.

Signed-off-by: Arunprasad Rajkumar ar.arunprasad@gmail.com

This is how it looks on Promscale dashboard

TODO

• [ ] Decide on bucket size - Current bucket size is inclusive of both metrics and traces.
• [x] Dashboard(Histogram vs Heat-map)
• [ ] Alerting? - I'm not sure how feasible it would be when considering low ingestion use cases.

Description

Merge requirements

Please take into account the following non-code changes that you may need to make with your PR:

• [x] CHANGELOG entry for user-facing changes
• [ ] Updated the relevant documentation

[arajkumar]

@niksajakovljevic @cevian @paulfantom I've also added heatmap panel to visualize newly added histogram. Please check.