Drop mcp.fields.bytes_saved metric

Remove the mcp.fields.bytes_saved counter. It is derivable on the
dashboard from the two remaining byte counters, since
sum(bytes_full) - sum(bytes_sent) equals the total saved at any rollup,
so emitting it separately is redundant. Keeping only bytes_full and
bytes_sent shrinks the emitted telemetry surface.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: tommaso-moro

pkg/github/fields_telemetry.go

@@ -13,11 +13,14 @@ import (
 // (a small fixed set of tool names) and `filtered` (a boolean). Unbounded values
 // such as repository, owner, user, the query, or the requested field list are
 // never used as tags.
+//
+// The realized savings (bytes_full - bytes_sent) is intentionally not emitted as
+// its own metric: it is derivable on the dashboard from the two byte counters,
+// since sum(bytes_full) - sum(bytes_sent) equals the total saved at any rollup.
 const (
-	metricFieldsToolCall   = "mcp.fields.tool_call"
-	metricFieldsBytesFull  = "mcp.fields.bytes_full"
-	metricFieldsBytesSent  = "mcp.fields.bytes_sent"
-	metricFieldsBytesSaved = "mcp.fields.bytes_saved"
+	metricFieldsToolCall  = "mcp.fields.tool_call"
+	metricFieldsBytesFull = "mcp.fields.bytes_full"
+	metricFieldsBytesSent = "mcp.fields.bytes_sent"
 )
 
 // recordFieldsUsage emits telemetry for a single call to a tool that supports
@@ -27,10 +30,9 @@ const (
 // Every call increments mcp.fields.tool_call tagged by tool and whether the
 // response was filtered, which yields the adoption rate (filtered / total). When
 // the response was filtered, it also records the unfiltered (fullBytes) and
-// returned (sentBytes) payload sizes plus their difference, which yields the
-// realized savings. Byte counters are only emitted for filtered calls so that
-// "percent saved" (bytes_saved / bytes_full) is computed over the population
-// where filtering actually applied.
+// returned (sentBytes) payload sizes. Byte counters are only emitted for
+// filtered calls so that "percent saved" (1 - bytes_sent / bytes_full) is
+// computed over the population where filtering actually applied.
 func recordFieldsUsage(ctx context.Context, deps ToolDependencies, tool string, filtered bool, fullBytes, sentBytes int) {
 	m := deps.Metrics(ctx)
 	if m == nil {
@@ -47,8 +49,6 @@ func recordFieldsUsage(ctx context.Context, deps ToolDependencies, tool string,
 	}
 
 	toolTag := map[string]string{"tool": tool}
-	saved := max(fullBytes-sentBytes, 0)
 	m.Counter(metricFieldsBytesFull, toolTag, int64(fullBytes))
 	m.Counter(metricFieldsBytesSent, toolTag, int64(sentBytes))
-	m.Counter(metricFieldsBytesSaved, toolTag, int64(saved))
 }

pkg/github/fields_telemetry_test.go

@@ -97,10 +97,6 @@ func Test_recordFieldsUsage_Filtered(t *testing.T) {
 	sent, ok := rec.counter(metricFieldsBytesSent)
 	require.True(t, ok)
 	assert.Equal(t, int64(30), sent.value)
-
-	saved, ok := rec.counter(metricFieldsBytesSaved)
-	require.True(t, ok)
-	assert.Equal(t, int64(70), saved.value)
 }
 
 func Test_recordFieldsUsage_NotFiltered(t *testing.T) {
@@ -115,21 +111,10 @@ func Test_recordFieldsUsage_NotFiltered(t *testing.T) {
 	// No byte counters are emitted when the response was not filtered.
 	_, ok = rec.counter(metricFieldsBytesFull)
 	assert.False(t, ok)
-	_, ok = rec.counter(metricFieldsBytesSaved)
+	_, ok = rec.counter(metricFieldsBytesSent)
 	assert.False(t, ok)
 }
 
-func Test_recordFieldsUsage_ClampsNegativeSavings(t *testing.T) {
-	deps, rec := depsWithRecordingMetrics(t, BaseDeps{})
-
-	// sent larger than full should never yield negative savings.
-	recordFieldsUsage(context.Background(), deps, "get_file_contents", true, 10, 25)
-
-	saved, ok := rec.counter(metricFieldsBytesSaved)
-	require.True(t, ok)
-	assert.Equal(t, int64(0), saved.value)
-}
-
 func Test_recordFieldsUsage_NilExporterDoesNotPanic(t *testing.T) {
 	// BaseDeps with no Obsv falls back to a noop sink rather than panicking.
 	assert.NotPanics(t, func() {

pkg/github/repositories_test.go

@@ -610,10 +610,7 @@ func Test_GetFileContents_DirectoryFieldsTelemetry(t *testing.T) {
 	require.True(t, ok)
 	sent, ok := rec.counter(metricFieldsBytesSent)
 	require.True(t, ok)
-	saved, ok := rec.counter(metricFieldsBytesSaved)
-	require.True(t, ok)
 	assert.Greater(t, full.value, sent.value, "filtering should remove bytes")
-	assert.Equal(t, full.value-sent.value, saved.value)
 }
 
 func Test_GetFileContents_IFC_InsidersMode(t *testing.T) {

pkg/github/search_test.go

@@ -631,10 +631,7 @@ func Test_SearchCode_FieldsTelemetry(t *testing.T) {
 		require.True(t, ok)
 		sent, ok := rec.counter(metricFieldsBytesSent)
 		require.True(t, ok)
-		saved, ok := rec.counter(metricFieldsBytesSaved)
-		require.True(t, ok)
 		assert.Greater(t, full.value, sent.value, "filtering should remove bytes")
-		assert.Equal(t, full.value-sent.value, saved.value)
 	})
 
 	t.Run("unfiltered call records adoption only", func(t *testing.T) {