refactor(test_consumption_metrics): split for pageserver and proxy

[koivunej]

With the addition of the "stateful event verification" the test_consumption_metrics.py is now too crowded. Split it up for pageserver and proxy.

Split from #5297. Next up: #5325.

[github-actions]

2484 tests run: 2363 passed, 0 failed, 121 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_partial_evict_tenant: debug
• test_wal_lagging: release

Postgres 14

• test_download_remote_layers_api[local_fs]: debug
• test_get_tenant_size_with_multiple_branches: debug

Code coverage (full report)

• functions: 53.0% (7751 of 14613 functions)
• lines: 81.1% (45259 of 55836 lines)

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_{The comment gets automatically updated with the latest test results
9dce5af at 2023-09-16T12:24:15.615Z :recycle:}