Set statement_timeout at session level & use reset_val as setting when source is session

[lu-zhengda]

What does this PR do?

This PR

• Sets statement_timeout at session level instead of connection level after database connection is established
• Use reset_val in pg_settings query when source equals to session.
  

Motivation

https://datadoghq.atlassian.net/browse/DBMON-3728
Prior to the change, the statement_timeout setting collected from pg_settings is set by datadog integration connection (default to 5s). This does not reflect the database level statement_timeout setting and is often misleading. In this PR, we set the statement_timeout after the connection is established. This way we can do a simple check case when source = 'session' then reset_val else setting end as setting to retrieve the server level setting.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.48%. Comparing base (9503e88) to head (bdd7f37).
Report is 3 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
postgres	93.06% <87.50%> (+0.09%)	⬆️
presto	?
solr	?

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[github-actions]

Test Results

   16 files     16 suites   20m 26s ⏱️
  282 tests   280 ✅   2 💤 0 ❌
2 264 runs  2 149 ✅ 115 💤 0 ❌

Results for commit bdd7f37.