refactor(consumption_metrics): prereq refactorings, tests

[koivunej]

Split off from #5297.

There should be no functional changes here:

• refactor tenant metric "production" like previously timeline, allows unit testing, though not interesting enough yet to test
• introduce type aliases for tuples
• extra refactoring for collect, was initially thinking it was useful but will do a inline later
• shorter binding names
• support for future allocation reuse quests with IdempotencyKey
• move code out of tokio::select to make it rustfmt-able
• generification, allow later replacement of &'static str with enum
• add tests that assert sent event contents exactly

Next up are two non-conflicting:

• feat(consumption_metrics): remove event deduplication support #5316
• fix(consumption_metrics): exp backoff retry #5317

[github-actions]

2478 tests run: 2357 passed, 0 failed, 121 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_crafted_wal_end[last_wal_record_crossing_segment]: release

Postgres 14

• test_download_remote_layers_api[local_fs]: debug, release
• test_get_tenant_size_with_multiple_branches: release

Code coverage (full report)

• functions: 53.0% (7673 of 14483 functions)
• lines: 81.0% (44856 of 55401 lines)

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_{The comment gets automatically updated with the latest test results
733e572 at 2023-09-15T15:08:30.492Z :recycle:}