feat: listen/notify stats; fix: race condition in LISTEN

[levkk]

• feat: SHOW LISTENERS admin database stats on each pub/sub channel
• feat: OpenMetrics/OTEL for the same thing
• fix: race condition in LISTEN command handler when registering for the first time

Stats recorded:

• number of listeners
• number of messages received
• number of messages dropped

[codecov]

Codecov Report

❌ Patch coverage is 86.68478% with 49 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/stats/listeners.rs	66.12%	21 Missing ⚠️
pgdog/src/backend/pub_sub/listener.rs	95.15%	8 Missing ⚠️
pgdog/src/stats/http_server.rs	0.00%	7 Missing ⚠️
pgdog/src/admin/show_listeners.rs	83.33%	4 Missing ⚠️
pgdog/src/admin/parser.rs	71.42%	2 Missing ⚠️
pgdog/src/backend/pool/connection/mod.rs	0.00%	2 Missing ⚠️
pgdog/src/backend/pub_sub/client.rs	96.72%	2 Missing ⚠️
pgdog/src/stats/otel_exporter.rs	0.00%	2 Missing ⚠️
pgdog/src/backend/pool/shard/mod.rs	0.00%	1 Missing ⚠️

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