fix(perf): Improve contrast on span sample chart markline (#57024)

Similar to #50658. Improves contrast/readability on the span sample
chart.

- Increase font size
- Change colour and background

static/app/views/starfish/views/spanSummaryPage/sampleList/durationChart/index.tsx

@@ -107,9 +107,11 @@ function DurationChart({
       },
       emphasis: {disabled: true},
       label: {
-        fontSize: 11,
         position: 'insideEndBottom',
         formatter: () => 'Average',
+        fontSize: 14,
+        color: theme.chartLabel,
+        backgroundColor: theme.chartOther,
       },
     },
   };