OpenTelemetry metrics for collection dispatch and pagination

[dahlia]

Summary

Add OpenTelemetry metrics for ActivityPub collection dispatch and pagination, including request counts, dispatch duration, and collection page sizes.

Current state

Fedify already creates spans for activitypub.dispatch_collection and activitypub.dispatch_collection_page. The OpenTelemetry documentation also lists attributes such as activitypub.collection.total_items, fedify.collection.cursor, and fedify.collection.items.

Those spans help debug a single collection request. Operators still need aggregate metrics to see whether collection endpoints are slow, whether pagination is producing unexpectedly large pages, or whether custom collection dispatchers are a source of latency.

Proposed solution

Once #619 adds metrics support, add metrics around collection and collection-page dispatch.

Proposed instruments:

• activitypub.collection.request: counter, incremented when Fedify dispatches a collection or collection page.
• activitypub.collection.dispatch.duration: histogram, recording dispatcher duration in milliseconds.
• activitypub.collection.page.items: histogram, recording the number of items returned in a collection page.
• activitypub.collection.total_items: histogram, recording reported collection size where available.

Proposed attributes:

• activitypub.collection.kind: followers, following, liked, featured, outbox, or custom, where Fedify can classify the collection.
• activitypub.collection.page: true or false.
• activitypub.collection.result: served, not_found, not_acceptable, or error.
• fedify.collection.dispatcher: built_in or custom.

Do not include actor identifiers, collection IDs, cursor values, object IDs, or route parameter values as metric attributes.

Scope

• Instrument built-in and custom collection dispatcher paths in @fedify/fedify.
• Record page item counts where Fedify has the page items in memory.
• Record total item counts where the dispatcher reports them without extra work.
• Keep remote collection fetching and conversation backfill out of scope.
• Update docs/manual/opentelemetry.md with metric names, units, and cardinality guidance.

Acceptance criteria

• Collection and collection-page dispatch counts are emitted for successful and failed requests.
• Dispatcher duration is recorded for collection and collection-page paths.
• Page item count histograms are recorded without exposing item IDs or cursors.
• Total item count histograms are recorded only when the value is already available.
• Tests cover at least one collection and one collection page path.
• Documentation explains which collection kinds are classified and how custom collections are labeled.

Open questions

• Should collection kind use existing dispatcher names or a smaller normalized taxonomy?
• Should activitypub.collection.total_items be a histogram, gauge, or span-only attribute?
• Should collection metrics include outbox serving metrics, or should outbox be tracked only through ActivityPub delivery metrics?