[SLOP(claude-opus-4-8)] feat(rivetkit-core): add serde duration and size metrics - #5322

Stack for rivet-dev/rivet

• #5552 [slopfix] fix(universaldb): match fdb conflict semantics in …
• #5551 [slopfix] fix(universaldb,util,pegboard-envoy): index dedup …
• #5398 feat(kitchen-sink): add churnDb actor and seed driver for co …
• #5397 chore: Clean up
• #5382 [slopfix] docs(self-hosting): correct postgres config format …
• #5356 [slopfix] fix(pegboard): decrement kv_storage_used metric on …
• #5354 [slopfix] refactor(universaldb,ups): single/multi-node udb o …
• #5353 [slopfix] perf(universaldb): fold postgres drain_batch into …
• #5342 [SLOP(claude-opus-4-8-high)] perf(universaldb): batch leader …
• #5341 [slopfix] docs(self-hosting): promote postgres from experime …
• #5340 [slopfix] fix(self-host): repair dev compose generation, kit …
• #5337 [SLOP(claude-opus-4-8-high)] feat(universaldb): graceful pos …
• #5336 [slopfix] test(universaldb): postgres leader failover + abor …
• #5335 [SLOP(claude-opus-4-8-high)] feat(ups): table-backed postgre …
• #5329 [SLOP(claude-opus-4-8-high)] feat(universaldb): postgres lea …
• #5327 [SLOP(claude-opus-4-8)] feat(pegboard-envoy): rate-limit env …
• #5326 [SLOP(claude-opus-4-8)] feat(util): add rate limiter primiti …
• #5322 [SLOP(claude-opus-4-8)] feat(rivetkit-core): add serde durat … 👈
• #5321 Add observation for serde_bare
• #5320 Add observation and ser/de metrics for serde calls
• #5314 [SLOP(claude-opus-4-8-medium)] chore: add profiling cargo pr …
• #5306 [SLOP(claude-opus-4-8)] fix(kitchen-sink): serve built front …
• main

Get stack: forklift get 5322
Push local edits: forklift submit
Merge when ready: forklift merge 5322

_{change nqkrytvr}

Review

Adds duration/size Prometheus histograms for serde hot paths in rivetkit-core, following the same LazyLock collector + register_metric-with-warn-on-duplicate pattern already used by rivet-envoy-client (engine/sdks/rust/envoy-client/src/metrics.rs), and reuses the exact byte-size buckets from the engine-side SERIALIZE_SIZE/DESERIALIZE_SIZE metrics (engine/packages/util/src/metrics.rs). Labels (format, location) are bounded/code-defined, consistent with the metrics cardinality rules in CLAUDE.md. crate::time::Instant is used correctly for wasm compatibility, and rivet_metrics is already an unconditional dependency of this crate (see metrics_endpoint.rs), so no new wasm-compat risk.

Findings

1. Inconsistent instrumentation coverage in http.rs. decode_http_action_args/encode_http_action_response are wrapped with serde_metrics::measure_*, but the structurally identical queue counterparts, decode_http_queue_request (registry/http.rs:878) and encode_http_queue_response (registry/http.rs:957), are left uninstrumented. If the goal is observability into serde hot paths, the queue send/response path is a real gap, not just a style inconsistency, worth wrapping in the same change or filing a follow-up.

2. No test coverage for the new module. serde_metrics.rs has no accompanying tests, even though this crate already has an established pattern for exactly this kind of thing in tests/metrics.rs (e.g. duplicate_metric_registration_uses_noop_fallback, actor_startup_duration_metrics_render, which render global metrics and assert on labeled values). A similar test asserting rivetkit_serialize_size/rivetkit_deserialize_size/*_duration_seconds get recorded with the right format/location labels (and that failed deserialization still records size) would catch label typos and regressions.

3. Minor doc nit: the module doc says it "mirror[s] the engine-side serde observability," but the engine side (engine/packages/util/src/metrics.rs) only has SERIALIZE_SIZE/DESERIALIZE_SIZE (size only, no duration histograms). Not a blocker, just slightly overstates the parity.

4. PR title includes [SLOP(claude-opus-4-8)]. Per CLAUDE.md, commit/PR titles must be pure conventional commits with no agent/model markers. Worth stripping before merge if this was not intentional tooling metadata.

Nothing else stood out. The closures in decode_http_action_args/encode_http_action_response correctly preserve existing error propagation via ?, persist.rs callers all pass static bounded label strings (no unbounded cardinality), and hard-tab formatting is intact.