Refactor metrics emitter interface

Use a single metrics emitter abstraction for gauge, count, and histogram samples so metric helpers share one testable client contract.

Author: forge33

go/base/context.go

@@ -238,7 +238,7 @@ type MigrationContext struct {
 	AbortError error
 	abortMutex *sync.Mutex
 
-	Metrics metrics.MemStatsGaugeEmitter
+	Metrics metrics.Emitter
 
 	OriginalTableColumnsOnApplier    *sql.ColumnList
 	OriginalTableColumns             *sql.ColumnList

go/logic/migrator_test.go

@@ -419,6 +419,12 @@ func (s *progressGaugeSpy) Gauge(name string, value float64, tags ...string) {
 	s.tags = append(s.tags, append([]string(nil), tags...))
 }
 
+func (s *progressGaugeSpy) Count(name string, value int64, tags ...string) {
+}
+
+func (s *progressGaugeSpy) Histogram(name string, value float64, tags ...string) {
+}
+
 func TestReportStatusEmitsProgressGaugesEveryTick(t *testing.T) {
 	spy := &progressGaugeSpy{}
 	ctx := base.NewMigrationContext()

go/metrics/binlog_backlog.go

@@ -7,7 +7,7 @@ package metrics
 
 // EmitBinlogBacklogGauges emits apply-events queue depth gauges (namespace is applied by the client):
 // gh_ost.binlog.backlog_size, gh_ost.binlog.backlog_capacity, gh_ost.binlog.backlog_utilization.
-func EmitBinlogBacklogGauges(emit MemStatsGaugeEmitter, backlogSize, backlogCapacity int) {
+func EmitBinlogBacklogGauges(emit Emitter, backlogSize, backlogCapacity int) {
 	if emit == nil {
 		return
 	}

go/metrics/client.go

@@ -21,9 +21,11 @@ type Client struct {
 	sd *statsd.Client
 }
 
-// MemStatsGaugeEmitter is implemented by *Client; used for tests without UDP.
-type MemStatsGaugeEmitter interface {
+// Emitter is implemented by *Client; used for tests without UDP.
+type Emitter interface {
 	Gauge(name string, value float64, tags ...string)
+	Count(name string, value int64, tags ...string)
+	Histogram(name string, value float64, tags ...string)
 }
 
 // NewClient connects to addr for StatsD. If addr is empty, returns Noop and nil error.
@@ -65,6 +67,13 @@ func (c *Client) Count(name string, value int64, tags ...string) {
 	_ = c.sd.Count(name, value, tags, 1.0)
 }
 
+func (c *Client) Histogram(name string, value float64, tags ...string) {
+	if c.sd == nil {
+		return
+	}
+	_ = c.sd.Histogram(name, value, tags, 1.0)
+}
+
 // Close flushes buffered metrics; safe for Noop.
 func (c *Client) Close() error {
 	if c.sd == nil {

go/metrics/go_runtime.go

@@ -13,7 +13,7 @@ import (
 
 // EmitGoRuntimeGauges emits gh_ost.go_runtime.* gauges (namespace is applied by the client).
 // m and numGoroutine are typically from runtime.ReadMemStats and runtime.NumGoroutine.
-func EmitGoRuntimeGauges(emit MemStatsGaugeEmitter, m *runtime.MemStats, numGoroutine int) {
+func EmitGoRuntimeGauges(emit Emitter, m *runtime.MemStats, numGoroutine int) {
 	if emit == nil || m == nil {
 		return
 	}

go/metrics/go_runtime_test.go

@@ -24,6 +24,12 @@ func (g *gaugeSpy) Gauge(name string, value float64, tags ...string) {
 	g.tags = append(g.tags, append([]string(nil), tags...))
 }
 
+func (g *gaugeSpy) Count(name string, value int64, tags ...string) {
+}
+
+func (g *gaugeSpy) Histogram(name string, value float64, tags ...string) {
+}
+
 func TestEmitGoRuntimeGauges(t *testing.T) {
 	spy := &gaugeSpy{}
 	m := &runtime.MemStats{

go/metrics/lag.go

@@ -13,7 +13,7 @@ import "fmt"
 // These are point-in-time readings each status tick (not a distribution), so gauges are used
 // rather than histograms — DogStatsD histogram aggregation exposes count/max series that do not
 // match the log line lag values in Prometheus/Grafana.
-func EmitLagGauges(emit MemStatsGaugeEmitter, replicationLagSeconds, heartbeatLagSeconds float64, throttled bool) {
+func EmitLagGauges(emit Emitter, replicationLagSeconds, heartbeatLagSeconds float64, throttled bool) {
 	if emit == nil {
 		return
 	}

go/metrics/progress.go

@@ -7,7 +7,7 @@ package metrics
 
 // EmitProgressGauges emits row-copy and DML progress gauges (namespace is applied by the client):
 // gh_ost.row_copy.rows_copied, gh_ost.row_copy.rows_estimate, gh_ost.dml.events_applied.
-func EmitProgressGauges(emit MemStatsGaugeEmitter, rowsCopied, rowsEstimate, dmlEventsApplied int64) {
+func EmitProgressGauges(emit Emitter, rowsCopied, rowsEstimate, dmlEventsApplied int64) {
 	if emit == nil {
 		return
 	}