Add metrics for indexer decode failures

[priscaenoch]

Problem

indexer/src/decoder.js falls back to a function name of "unknown" and emits raw_topics when it cannot recognize an event or its ABI. This fallback is silent: there is no counter or log that records how often decoding falls through, so there is no way to measure decoding coverage or detect a regression where a whole class of events stops decoding. Since human-readable decoding is the core value of the product, this blind spot matters.

What needs to be done

• Increment a metric each time the decoder falls back to the unknown/raw_topics path, labeled by contract and/or event where feasible.
• Emit a debug/structured log for fallbacks so they can be sampled and investigated.
• Expose the metric via the service's metrics endpoint so decoding coverage can be tracked over time.
• Document what a rising fallback rate indicates.

Files

• indexer/src/decoder.js
• indexer metrics/exposition and indexer/src/api.js

Acceptance deliverables

• Decode fallbacks are counted and observable.
• A rising fallback rate is visible via metrics.
• All CI checks pass; the change cannot be merged until CI is green.

Tests to pass

• Test: decoding an unrecognized event increments the fallback counter.
• Test: decoding a known event does not increment it.

[Ayinkx01]

Hi i would like to work on this issue please assign to me
I can do it perfectly

[DEV-DESIGN6]

Hello, I can solve this.

[samuel2926i39-art]

I’d like to take on this issue. I’m comfortable working with any codebase and writing tests, and I’m interested in improving the reliability of this project’s workflow

[grantfox-oss]

🦊 GrantFox — @samuel2926i39-art has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #51)
2. Your PR will be reviewed by the octraban maintainers

Good luck! Track your progress on GrantFox.