chore: wire telemetry into AsyncConnector, off by default

[enocom]

Part 8 of 9 in the built-in telemetry stack, splitting what was previously one ~3,200 line commit (#539) into reviewable pieces.

Based on #572 — review that one first; only the last commit here is new.

Unit tests, ruff check, ruff format --check and mypy are green at every commit in the stack, not just at the tip.

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Same wiring as the synchronous Connector: a recorder per instance, one
dial_count per dial with a status, dial_latency on success, and the
refresh caches reporting under the same instance.

enable_builtin_telemetry defaults to False here too. The default flips
for both connectors in a change of its own.

Two things differ from the synchronous path. asyncpg surfaces a single
error for the whole connect, so a PostgresError — the server answered and
rejected us — is classified as a user error and everything else as a TCP
error, rather than letting tcp_error become a catch-all that hides user
mistakes. And open_connections is decremented from an asyncpg termination
listener, which Connection._cleanup invokes at most once on both close()
and terminate().

Neither byte counts nor mdx_error are reported here: asyncpg exposes no
hook for observing bytes on the wire, and the async path performs no
metadata exchange.

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The stack (merges bottom to top):

1. refactor: extract _metadata_exchange helper #566 — refactor: extract _metadata_exchange helper
2. fix: do not mask dial errors with KeyError #567 — fix: do not mask dial errors with KeyError
3. fix: close the socket when the metadata exchange fails #568 — fix: close the socket when the metadata exchange fails
4. chore: add telemetry provider, recorders, and exporter #569 — chore: add telemetry provider, recorders, and exporter
5. chore: add InstrumentedSocket #570 — chore: add InstrumentedSocket
6. chore: record refresh metrics #571 — chore: record refresh metrics
7. chore: wire telemetry into Connector, off by default #572 — chore: wire telemetry into Connector, off by default
8. chore: wire telemetry into AsyncConnector, off by default #573 — chore: wire telemetry into AsyncConnector, off by default ← you are here
9. feat: enable built-in telemetry by default #574 — feat: enable built-in telemetry by default

Group A (1–3) is independent of telemetry and could ship on its own, and carries no breaking change: dial failures are classified for the metric by tagging the exception in #572, not by changing the type callers see. Group B (4–8) is inert because enable_builtin_telemetry defaults to False throughout. #574 flips that default, so reverting it alone turns the whole feature off.