feat(feed): add duration metrics for feed updates, retrieval, and availability in v1 and v2

[akrem-chabchoub]

Feed check metrics

Adds Prometheus metrics to the feed checks (feed-v1 and feed) so their runs are observable like the other beekeeper checks.

Metrics

• feed_update_duration_seconds – time for each feed update (upload + UpdateFeedWithReference / UpdateFeedWithRootChunk)
• feed_retrieval_duration_seconds – time from FindFeedUpdate to DownloadFileBytes completion

Changes

• New pkg/check/feed/metrics.go with two histograms
• CheckV1 and CheckV2 implement metrics.Reporter and register collectors with the check runner
• Subsystems: check_feed_v1 for feed-v1, check_feed_v2 for feed

[gacevicljubisa]

Most of the checks in the codebase are tracking successes and failures, so maybe add counter metrics:

• FeedUpdatesTotal
• FeedUpdateErrorsTotal
• FeedRetrievalsTotal
• FeedRetrievalErrorsTotal

[akrem-chabchoub]

Most of the checks in the codebase are tracking successes and failures, so maybe add counter metrics:

• FeedUpdatesTotal
• FeedUpdateErrorsTotal
• FeedRetrievalsTotal
• FeedRetrievalErrorsTotal

Yes I agree, but for the purpose of this PR is to have performance metrics.
I can do it in another separate PR on top of this one, wdyt ?

[gacevicljubisa]

Also,

Most of the checks in the codebase are tracking successes and failures, so maybe add counter metrics:

• FeedUpdatesTotal
• FeedUpdateErrorsTotal
• FeedRetrievalsTotal
• FeedRetrievalErrorsTotal

Yes I agree, but for the purpose of this PR is to have performance metrics. I can do it in another separate PR on top of this one, wdyt ?

If there is an issue for this please attach it and create separate PR then. If there is not, do how you prefer.

[gacevicljubisa]

@akrem-chabchoub do we need to measure the checkAvailability flow, when RootRef is defined? I see that this is not covered.