database_observability: cache pg sample and wait events before sending finalised entries (#4523)

* draft cache

* tests

* cover more scenarios with tests

* refactor samples code

* further reorganize code

* lint fix

* changelog

* reword changelog

* do not round wait times, filter rows to only client backend type

* removing backend_time from samples, adding pid and xids to the wait events for easier matching

* Revert "do not round wait times, filter rows to only client backend type"

This reverts commit d908305.

* round time to nanoseconds

* address review comments

* remove redundant comments

* add back state field to samples

Author: gaantunes

CHANGELOG.md

@@ -25,6 +25,7 @@ Main (unreleased)
 - (_Experimental_) Additions to experimental `database_observability.postgres` component:
   - `explain_plans` added the explain plan collector (@rgeyer)
   - add `user` field to wait events within `query_samples` collector (@gaantunes)
+  - rework the query samples collector to buffer per-query execution state across scrapes and emit finalized entries (@gaantunes)
 
 - Add `otelcol.exporter.googlecloudpubsub` community component to export metrics, traces, and logs to Google Cloud Pub/Sub topic. (@eraac)