ingotctl, HTTP layer, and self-instrumentation

Add cmd/ingotctl with four subcommands: blocks (list with stats),
inspect (series/postings dump), chunks (decode raw samples by ref), and
fsck (CRC and index integrity validation across all blocks).

Add cmd/ingothttp with JSON query endpoints: /api/v1/query_range
(Prometheus-style matrix response), /api/v1/read (matcher-based read
requests), and /epi/v1/status (DB stats snapshot). Uses JSON instead of
protobuf to maintain zero dependencies.

Add self-instrumentation via the normal Appender path so metrics are
queryable with the same API: ingot_head_series,
ingot_head_chunks_active, ingot_blocks_total, ingot_compactions_total,
ingot_wal_fsync_duration_seconds.

Supporting changes: block.Validate() and block.ReadMeta() exports,
Head.Stats() for series/chunk counts, WAL.LastSyncDuration() with timed
fsync tracking, DB.Stats() for the HTTP status endpoint.

metrics.go

@@ -0,0 +1,68 @@
+package ingot
+
+import (
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+// Self-instrumentation metric names.
+const (
+	MetricHeadSeries         = "ingot_head_series"
+	MetricHeadChunksActive   = "ingot_head_chunks_active"
+	MetricBlocksTotal        = "ingot_blocks_total"
+	MetricCompactionsTotal   = "ingot_compactions_total"
+	MetricWALFsyncDurationS  = "ingot_wal_fsync_duration_seconds"
+)
+
+// metricsRefs caches series refs for self-instrumentation metrics.
+type metricsRefs struct {
+	headSeries       uint64
+	headChunksActive uint64
+	blocksTotal      uint64
+	compactionsTotal uint64
+	walFsyncDuration uint64
+}
+
+// collectMetrics snapshots the DB's internal stats and writes them as
+// ingot series via the normal Appender path. Called periodically from
+// the compact loop.
+func (db *DB) collectMetrics() {
+	now := db.opts.clock()()
+
+	hs := db.head.Stats()
+	db.mu.RLock()
+	numBlocks := len(db.blocks)
+	db.mu.RUnlock()
+	compactions := db.compactionCount.Load()
+	walFsync := db.head.WALSyncDuration()
+
+	type metric struct {
+		name  string
+		value float64
+		ref   *uint64
+	}
+	metrics := []metric{
+		{MetricHeadSeries, float64(hs.NumSeries), &db.metricsR.headSeries},
+		{MetricHeadChunksActive, float64(hs.NumActiveChunks), &db.metricsR.headChunksActive},
+		{MetricBlocksTotal, float64(numBlocks), &db.metricsR.blocksTotal},
+		{MetricCompactionsTotal, float64(compactions), &db.metricsR.compactionsTotal},
+		{MetricWALFsyncDurationS, walFsync, &db.metricsR.walFsyncDuration},
+	}
+
+	app := db.Appender()
+	for _, m := range metrics {
+		ref := *m.ref
+		var ls []labels.Label
+		if ref == 0 {
+			ls = labels.FromStrings("__name__", m.name)
+		}
+		newRef, err := app.Append(ref, ls, now, m.value)
+		if err != nil {
+			// OOO rejection or other transient error — skip this cycle.
+			app.Rollback()
+			return
+		}
+		*m.ref = newRef
+	}
+	app.Commit()
+}
+