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]

4968 tests run: 4794 passed, 10 failed, 164 skipped (full report)

---

Failures on Postgres 17

• test_metrics_normal_work: release-x86-64, debug-x86-64, release-arm64

Failures on Postgres 16

• test_slots_and_branching: release-x86-64
• test_metrics_normal_work: release-x86-64, release-arm64

Failures on Postgres 15

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

Failures on Postgres 14

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

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_metrics_normal_work[release-pg14] or test_metrics_normal_work[release-pg14] or test_metrics_normal_work[release-pg15] or test_metrics_normal_work[release-pg15] or test_slots_and_branching[release-pg16] or test_metrics_normal_work[release-pg16] or test_metrics_normal_work[release-pg16] or test_metrics_normal_work[release-pg17] or test_metrics_normal_work[debug-pg17] or test_metrics_normal_work[release-pg17]"

Flaky tests (8)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_scrubber_physical_gc[4]: release-arm64

Postgres 16

• test_scrubber_physical_gc[4]: release-x86-64, release-arm64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Postgres 14

• test_scrubber_physical_gc[4]: release-arm64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
50268a7 at 2024-09-19T21:42:34.883Z :recycle:}