Collect statement metrics & samples from all databases on host

[djova]

What does this PR do?

Update the collection of postgres statement metrics & samples to automatically collect data for all databases on a host.

Changes:

• collection of statement metrics & samples now respects the dbstrict setting
• in order to be able to collect telemetry from multiple databases the statement sampler thread now maintains a collection pool with one connection per database
• added a new ignore_databases configuration to enable users to define which databases on the host will be ignored. It defaults to the same exclusion list that was previously hardcoded in the "instance metrics" query. This setting is now shared across the whole check (instance metrics, statement metrics, statement samples)

Follow-up to #8627

Motivation

• Simplify configuration for collection of statement metrics, samples & execution plans for users by enabling collection from all databases on a host with only a single configured "check instance." Previously users had to enumerate each database in a host separately.
• Ensure that collection of statement samples & plans respects the dbstrict setting

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[ofek]

Nice! Have we considered preventing all other collection from the instance that runs this feature?

[djova]

Have we considered preventing all other collection from the instance that runs this feature?

Summary from offline discussion: the intended deployment configuration for the postgres check is to have only a single check instance for each postgres host. That one check instance will collect data from all databases on the host by default. If a user decides to configure multiple check instances for different databases on the same host then it's expected that there will be duplicate data collected by the check instances. In that case the user will need to set dbstrict: true to restrict the check instance to collecting data only from the specific database configured for that check instance.