docs: metric used in performance comparisons not annotated

[msladecek]

Issue

In bar charts available at https://metosin.github.io/reitit/performance.html, it is not obvious at first glance which metric is being compared across different frameworks.

In the current version, it is not clear what the metric is, or if it's the same metric in all comparison charts.

Minimal fix proposal:

Adding the name of the metric at the y axis or in the chart's header would be a sufficient fix.

[miikka]

Fair point. @ikitommi, you made the charts, could you do this?