feat: add 'MultiTracer' to go doc & cover it

doc.go

@@ -175,7 +175,7 @@ notification is received or the context is canceled.
 
 Tracing and Logging
 
-pgx supports tracing by setting ConnConfig.Tracer.
+pgx supports tracing by setting ConnConfig.Tracer. To combine several tracers you can use the MultiTracer.
 
 In addition, the tracelog package provides the TraceLog type which lets a traditional logger act as a Tracer.
 

tracer.go

@@ -105,6 +105,8 @@ type TraceConnectEndData struct {
 	Err  error
 }
 
+// MultiTracer can combine several tracers into one.
+// You can use NewMultiTracer to automatically split tracers by interface.
 type MultiTracer struct {
 	QueryTracers    []QueryTracer
 	BatchTracers    []BatchTracer
@@ -113,6 +115,7 @@ type MultiTracer struct {
 	ConnectTracers  []ConnectTracer
 }
 
+// NewMultiTracer returns new MultiTracer from tracers with automatically split tracers by interface.
 func NewMultiTracer(tracers ...QueryTracer) *MultiTracer {
 	var t MultiTracer
 

tracer_test.go

@@ -566,3 +566,51 @@ func TestTraceConnect(t *testing.T) {
 	require.True(t, traceConnectStartCalled)
 	require.True(t, traceConnectEndCalled)
 }
+
+type testCopyTracer struct{}
+
+func (tt *testCopyTracer) TraceQueryStart(ctx context.Context, conn *pgx.Conn, data pgx.TraceQueryStartData) context.Context {
+	return ctx
+}
+
+func (tt *testCopyTracer) TraceQueryEnd(ctx context.Context, conn *pgx.Conn, data pgx.TraceQueryEndData) {
+}
+
+func (tt *testCopyTracer) TraceCopyFromStart(ctx context.Context, conn *pgx.Conn, data pgx.TraceCopyFromStartData) context.Context {
+	return ctx
+}
+
+func (tt *testCopyTracer) TraceCopyFromEnd(ctx context.Context, conn *pgx.Conn, data pgx.TraceCopyFromEndData) {
+}
+
+func TestNewMultiTracer(t *testing.T) {
+	t.Parallel()
+
+	fullTracer := &testTracer{}
+	copyTracer := &testCopyTracer{}
+
+	mt := pgx.NewMultiTracer(fullTracer, copyTracer)
+	require.Equal(
+		t,
+		&pgx.MultiTracer{
+			QueryTracers: []pgx.QueryTracer{
+				fullTracer,
+				copyTracer,
+			},
+			BatchTracers: []pgx.BatchTracer{
+				fullTracer,
+			},
+			CopyFromTracers: []pgx.CopyFromTracer{
+				fullTracer,
+				copyTracer,
+			},
+			PrepareTracers: []pgx.PrepareTracer{
+				fullTracer,
+			},
+			ConnectTracers: []pgx.ConnectTracer{
+				fullTracer,
+			},
+		},
+		mt,
+	)
+}