chore: CON-1695 Report more error metrics during IDKG payload building

[eichhorl]

Previously, there were some error cases in the IDKG payload builder where we only created a log message. With this PR we additionally increment the idkg_payload_errors metric, which is connected to a slack alert.

Additionally we remove some debug! level logs (as they won't show up anyway), and upgrade some other debug! level logs to be warn!.

[Copilot]

Pull request overview

This PR increases observability for IDKG payload building by incrementing the idkg_payload_errors metric in additional error paths and adjusting log levels to improve signal.

Changes:

• Add an IDkgPayloadMetricsOptionExt helper to increment payload error counters when metrics are optionally available.
• Wire optional payload metrics through payload builder helpers and reshare/pre-signature flows, incrementing idkg_payload_errors in more failure cases.
• Remove/upgrade several debug! logs to warn! (often with rate limiting).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
rs/consensus/idkg/src/utils.rs	Use the new optional-metrics helper to always increment the payload error metric when chain cache building fails.
rs/consensus/idkg/src/signer.rs	Promote transient signature-share validation logs from debug! to rate-limited warn!.
rs/consensus/idkg/src/pre_signer.rs	Promote transient dealing validation / signing logs from debug! to rate-limited warn!.
rs/consensus/idkg/src/complaints.rs	Promote transient complaint/opening validation logs from debug! to rate-limited warn!.
rs/consensus/idkg/src/payload_verifier.rs	Update tests/call sites for new optional metrics parameter plumbed through payload building helpers.
rs/consensus/idkg/src/payload_builder/resharing.rs	Add optional metrics + new error increments for reshare request initiation/completion paths.
rs/consensus/idkg/src/payload_builder/pre_signatures.rs	Add optional metrics + new error increments for missing key transcript and receiver-set mismatch paths; remove debug logs.
rs/consensus/idkg/src/payload_builder.rs	Thread optional payload metrics through helper functions and into resharing/pre-signature update calls.
rs/consensus/idkg/src/metrics.rs	Introduce IDkgPayloadMetricsOptionExt to safely increment payload error metrics from Option<&IDkgPayloadMetrics>.

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