fix: consumption metrics on restart

[koivunej]

Fixes: #5206
Cc: #5175 (loggings, will be enhanced in follow-up)

Write collected metrics to disk to recover previously sent metrics on restart.

Recover the previously collected metrics during startup, send them over at right time

• send cached synthetic size before actual is calculated
• when last_record_lsn rolls back on startup
  • stay at last sent written_size metric
  • send written_size_delta_bytes metric as 0

Add test support: stateful verification of events in python tests.

[github-actions]

2484 tests run: 2362 passed, 0 failed, 122 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_get_tenant_size_with_multiple_branches: debug

Code coverage (full report)

• functions: 53.0% (7748 of 14613 functions)
• lines: 81.1% (45257 of 55836 lines)

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_{The comment gets automatically updated with the latest test results
875e219 at 2023-09-15T23:44:50.469Z :recycle:}