New internal/metrics/batch.go adds the batch series to metrics.Set, registered
in New: BatchJobsTotal and BatchTargetsTotal counters (label "outcome", any
value outside the known set folded to "other" so a raw string never becomes a
label), BatchQueueDepth and BatchTargetsInflight gauges, BatchJobSeconds and
BatchTargetSeconds histograms (the existing request buckets) and
BatchResultBytes (1K,4K,16K,64K,256K,1M). The Observe…/Set… methods are
nil-receiver safe like ObservePDF and flow through the existing /metrics
handler; no new endpoint or Snapshot change.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
