feat(tcp): add praxis_tcp_connections_total counter metric

[tsisodia10]

Increment a Prometheus counter on every accepted TCP connection, labeled by listener name. Gives operators visibility into connection throughput per listener over time.

What does this PR do?

Adds a praxis_tcp_connections_total Prometheus counter that increments once per accepted TCP connection, labeled by listener name. This enables operators to compute connection rates (connections/sec) per listener in dashboards.

Changes:

• New protocol/src/tcp/metrics.rs module with record_tcp_connection_accepted()
• Counter incremented in process_new() after overload checks pass
• Example config: examples/configs/observability/tcp-connections-total.yaml
• Unit test + integration tests (counter increments, correct label, example forwarding)

Which issue(s) does this relate to?

Fixes #1025

Checklist

• Signed off all commits (git commit -s)
• Tests added or updated
• Documentation updated (if applicable)
• make lint && make test passes locally

Does this introduce a breaking change?

No. The counter is purely additive and only emitted when the admin interface is enabled.

[praxis-bot]

PR Review: feat(tcp): add praxis_tcp_connections_total counter metric

Clean, well-scoped addition of a Prometheus counter for TCP connection throughput. The implementation correctly follows established metrics patterns (is_recorder_installed() guard, SharedString labels, placement after overload checks). The listener_label extraction avoids a redundant listener_label_for call.

Test coverage is solid: unit test for the no-recorder path, integration test verifying counter value after 3 connections (>= 3), and two example config tests exercising counter emission and traffic forwarding end-to-end.

Severity	Count
Critical	0
Large	0
Medium	1

1 inline comment posted.

[aslakknutsen]

Docs covered in #1036

[aslakknutsen]

Hmm.. I think this was just moved to Draft due to the policy change.

@tsisodia10 Could you resolve the conflicts?

[praxis-bot]

PR Review

Re-review: new commits since last review (2026-08-25).

Summary: The new commits introduced a merge from main that appears to have corrupted protocol/src/tcp/metrics.rs. The function record_tcp_connection_accepted is missing its closing brace, and duplicate SPDX headers and import lines were introduced. CI has only run metadata checks (DCO, commit messages), not build/lint/test, so these compilation errors are uncaught.

Overall: The PR cannot compile in its current state. Needs fixes before further review.

Severity	Count
Critical	1
Large	0
Medium	2

[shaneutt]

Appreciate a patch that is mostly tests. 🫡

Thank you for the PR 🖖