# P6 task 04: `sift_batch_*` series in `internal/metrics`

**Branch:** `p6` (`git switch p6`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `metrics: sift_batch_* series (jobs, targets, depth, in-flight, durations, bytes)`

Add the batch series to `internal/metrics` so the caps (32 targets, 1 job/caller, 8 jobs total)
and the byte budget can be re-sized from observed data. **This task touches only
`internal/metrics`**: task 06 wires these methods to the runner's `batch.Hooks`.

## Series (names and label values exact — the given test asserts these)

| Metric | Type | Labels / meaning |
|---|---|---|
| `sift_batch_jobs_total` | counter | `outcome` ∈ `created`, `reused`, `done`, `partial`, `failed`, `cancelled`, `restart` |
| `sift_batch_targets_total` | counter | `outcome` ∈ `ok`, `blocked`, `timeout`, `busy`, `upstream_4xx`, `upstream_5xx`, `unsupported`, `too_long`, `cancel` |
| `sift_batch_queue_depth` | gauge | live jobs (`queued`+`running`) |
| `sift_batch_targets_inflight` | gauge | targets currently being fetched |
| `sift_batch_job_seconds` | histogram | wall time from submit to terminal status |
| `sift_batch_target_seconds` | histogram | wall time per target |
| `sift_batch_result_bytes` | histogram | stored bytes per target result |

Classifying an error into a target outcome is task 06's job (it has the `*fetch.Error`); this
package only records the label it is given.

## Public surface (exact names — the given test compiles against them)

```go
// fields added to metrics.Set (registered in New)
BatchJobsTotal       *prometheus.CounterVec // label "outcome"
BatchTargetsTotal    *prometheus.CounterVec // label "outcome"
BatchQueueDepth      prometheus.Gauge
BatchTargetsInflight prometheus.Gauge
BatchJobSeconds      prometheus.Histogram
BatchTargetSeconds   prometheus.Histogram
BatchResultBytes     prometheus.Histogram

// methods (nil-guarded, like ObservePDF)
func (s *Set) ObserveBatchJob(outcome string)
func (s *Set) ObserveBatchTarget(outcome string)
func (s *Set) SetBatchQueueDepth(n int)
func (s *Set) SetBatchTargetsInflight(n int)
func (s *Set) ObserveBatchJobSeconds(d time.Duration)
func (s *Set) ObserveBatchTargetSeconds(d time.Duration)
func (s *Set) ObserveBatchResultBytes(n int)
```

Both `outcome` counters fold any value outside their known set to `other` — the same "a raw string
never becomes a label" guard as `ObservePDF`. Job outcomes: `created, reused, done, partial,
failed, cancelled, restart`. Target outcomes: `ok, blocked, timeout, busy, upstream_4xx,
upstream_5xx, unsupported, too_long, cancel`.

## Rules

1. A new file `internal/metrics/batch.go` adds the series to `metrics.Set` (registered in `New`),
   with `Observe…`/`Set…` methods mirroring `ObservePDF`/`ObserveDigest`, nil-receiver safe.
2. Both `outcome` counters fold any value outside their known set to `other`: a raw string
   never becomes a label.
3. The series are exposed through the existing `/metrics` and `/metrics.json` paths: no new
   endpoint. `Snapshot`/`writeMetricsJSON` do not need to list them.
4. Histogram buckets: seconds as the existing request histograms; bytes
   `1K, 4K, 16K, 64K, 256K, 1M`.

## Given

Copy `docs/plans/p6/_files/metrics/batch_test.go` → `internal/metrics/batch_test.go`. It drives the `Observe…`
methods, serves the `/metrics` handler (as `pdf_test.go` does), and asserts the exact series names,
label values, gauge values, histogram counts, and that a raw outcome string is folded to `other`.

## Steps

- [ ] **1.** `git switch p6`; copy the given test. **2. See it fail.**
- [ ] **3.** `internal/metrics/batch.go` and the registration in `New`.
- [ ] **4.** `gofmt -w`; `go test -race -count=3 ./internal/metrics/` → `ok`. **5.** `make gate`.
- [ ] **6.** Row `p6/04-metrics`; commit.

```sh
git add internal/metrics docs/implementer-log.md
git commit -F .state/commit-msg.txt
```

## Done when

- The given and existing metrics tests pass under `-race -count=3`; `make gate` ok; the given file is byte-identical.