pageserver: add counters for started smgr/getpage requests

[problame]

After this PR

curl localhost:9898/metrics | grep smgr_ | grep start

pageserver_smgr_query_started_count{shard_id="0000",smgr_query_type="get_page_at_lsn",tenant_id="...",timeline_id="..."} 0
pageserver_smgr_query_started_global_count{smgr_query_type="get_db_size"} 0
pageserver_smgr_query_started_global_count{smgr_query_type="get_page_at_lsn"} 0
pageserver_smgr_query_started_global_count{smgr_query_type="get_rel_exists"} 0
pageserver_smgr_query_started_global_count{smgr_query_type="get_rel_size"} 0
pageserver_smgr_query_started_global_count{smgr_query_type="get_slru_segment"} 0

We instantiate the per-tenant counter only for get_page_at_lsn.

[github-actions[bot]]

5046 tests run: 4881 passed, 1 failed, 164 skipped (full report)

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Failures on Postgres 17

• test_storage_controller_heartbeats[failure4]: release-x86-64

  # Run all failed tests locally:
  scripts/pytest -vv -n $(nproc) -k "test_storage_controller_heartbeats[release-pg17-failure4]"

  Flaky tests (7)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64, release-x86-64

• test_readonly_node_gc: debug-x86-64

• test_subscriber_restart: debug-x86-64

• test_pageserver_lsn_wait_error_safekeeper_stop: debug-x86-64

  Postgres 14

• test_scrubber_physical_gc[4]: release-arm64

• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.9% (7430 of 23303 functions)
• lines: 49.9% (59819 of 119956 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
90b23405d17e36048d3bb64e314067f397803f1b at 2024-09-20T13:18:13.749Z :recycle:}