fix: prevent Gateway API key exposure through metrics

[aicyberg]

Summary

• remove Graph Gateway API keys from Prometheus labels
• add a regression test proving encoded metrics contain no credential data
• default the unauthenticated metrics listener to 127.0.0.1
• document protection requirements for an explicit non-loopback bind

Why

In SSE mode, the current metric label sets include the raw Gateway API key.
The metrics listener also defaults to 0.0.0.0:9091 without authentication,
so a reachable endpoint can expose active credentials. Credential labels also
create unnecessary high-cardinality time series.

This change keeps operational dimensions (tool_name, endpoint_type, and
status) while removing credential material entirely. No real credential was
used or accessed during validation.

Validation

• cargo fmt --check
• cargo test (six tests)
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check