fix(mixed-timeseries): derive per-series metric from label map to stop duplicating first metric (#37921)

When Query A (or B) on a Mixed Chart has multiple metrics plus at least
one Group By dimension, the display-name builder in transformProps.ts
always prepended metrics[0]'s label: the entryName.includes(metricPart)
guard is false for every series belonging to metrics[1+], so those
series were renamed to e.g. 'score_one, score_two, A' instead of
'score_two, A' — the 'first metric duplicated in legend/tooltip'
symptom reported in #37921 (residual follow-up to #37055).

Derive each series' metric from its label_map tuple
([metric, ...dimensions]) instead, matching what the per-series
formatter lookup already does. Single-element tuples (no metric part)
fall back to the first metric, preserving existing behavior for
single-metric charts and showQueryIdentifiers naming. Applied to both
the Query A and Query B paths.

Includes the regression test originally landed red on this branch,
verified red on master without the fix and green with it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

superset-frontend/plugins/plugin-chart-echarts/test/MixedTimeseries/transformProps.test.ts

@@ -1161,3 +1161,82 @@ test('x-axis dedup keeps the forced min label when the endpoints format identica
 
   expect(formatter(min)).toBe('May');
 });
+
+test('regression #37921: multi-metric Query A with groupby does not duplicate first metric in series names', () => {
+  // Regression test for https://github.com/apache/superset/issues/37921
+  // ("Residual" follow-up to #37055).
+  //
+  // When Query A has multiple metrics + at least one Group By dimension,
+  // the display-name builder in transformProps.ts used to prepend the FIRST
+  // metric's display name to every series that didn't literally contain it:
+  //   name: `${MetricDisplayNameA}, ${entryName}`
+  // For series belonging to the *second* metric, this produced a
+  // cross-contaminated label like `score_one, score_two, A` — the
+  // user-visible "first metric duplicated" symptom in the legend / tooltip.
+  // The fix derives each series' metric from its label-map tuple instead.
+  const multiMetricRows = [
+    {
+      'score_one, A': 1,
+      'score_one, B': 2,
+      'score_two, A': 3,
+      'score_two, B': 4,
+      ds: 599616000000,
+    },
+    {
+      'score_one, A': 5,
+      'score_one, B': 6,
+      'score_two, A': 7,
+      'score_two, B': 8,
+      ds: 599916000000,
+    },
+  ];
+  const multiMetricLabelMap = {
+    ds: ['ds'],
+    'score_one, A': ['score_one', 'A'],
+    'score_one, B': ['score_one', 'B'],
+    'score_two, A': ['score_two', 'A'],
+    'score_two, B': ['score_two', 'B'],
+  };
+
+  const queryAData = createTestQueryData(multiMetricRows, {
+    label_map: multiMetricLabelMap,
+  });
+  // Query B keeps the existing single-metric shape — the bug is on
+  // Query A's path so we just need a valid Query B alongside.
+  const queryBData = createTestQueryData(defaultQueryRows, {
+    label_map: defaultLabelMap,
+  });
+
+  const chartProps = createEchartsTimeseriesTestChartProps<
+    EchartsMixedTimeseriesFormData,
+    EchartsMixedTimeseriesProps
+  >({
+    ...MIXED_TIMESERIES_CHART_PROPS_DEFAULTS,
+    defaultQueriesData: [queryAData, queryBData],
+    formData: {
+      ...formData,
+      metrics: ['score_one', 'score_two'],
+      groupby: ['category'],
+    },
+    queriesData: [queryAData, queryBData],
+  });
+  const transformed = transformProps(chartProps);
+
+  const queryASeriesNames = (transformed.echartOptions.series as any[])
+    .map((s: any) => String(s.name))
+    .filter((n: string) => n.includes('score_'));
+
+  // Each (metric, dim_value) combo from Query A should appear exactly once
+  // with the *correct* metric prefix — not the first-metric-prepended-to-
+  // everything-else form.
+  expect(queryASeriesNames).toContain('score_one, A');
+  expect(queryASeriesNames).toContain('score_one, B');
+  expect(queryASeriesNames).toContain('score_two, A');
+  expect(queryASeriesNames).toContain('score_two, B');
+
+  // And explicitly: no series name should contain *both* metric names —
+  // that's the smoking gun for the duplication bug.
+  for (const name of queryASeriesNames) {
+    expect(name).not.toMatch(/score_one,\s+score_two/);
+  }
+});