* PG: Directly run ANALYZE for the test_analyze_progress

Tests are currently using `vacuum analyze` to check metrics reporting
analyze progress. If the vacuum phase takes more than 1s, the test will
fail as the wait_for_value will exit and only a running vacuum will be
present when metrics are collected.
To avoid flakiness, we skip the vacuum phase and directly run an analyze.

* PG: Only test backend transaction age on test app

Don't check for metrics generated by datadog-agent app as background
queries could interfer. Instead, we only check for queries from the 'test'
app which should have a much more predictable behaviour.

* PG: Use dedicated application_name for vacuum and tx age tests

Queries could be left over by previous tests and possibly interfere with
following tests. Use a different application_name to avoid interference
between tests.