[tbmv3] Add TBMv3 accessiblity metric to system health

This change should not affect bots unless --experimental-tbmv3-metrics flag is supplied.
The metric is defined here: https://source.chromium.org/chromium/chromium/src/+/master:tools/perf/core/tbmv3/metrics/accessibility_metric.proto.

Change-Id: I600ce186b8f343f8c12d3d80a7d2f98bd83e3d8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339966
Commit-Queue: Julia Semavina <nuwanda@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Reviewed-by: Deep Roy <dproy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796061}

tools/perf/benchmarks/system_health.py

@@ -59,8 +59,9 @@ class _CommonSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
         'reportedByPageMetric',
         'tracingMetric',
         'umaMetric',
-        # Unless --experimentatil-tbmv3-metric flag is used, this metric does
-        # nothing.
+        # Unless --experimentatil-tbmv3-metric flag is used, the following tbmv3
+        # metrics do nothing.
+        'tbmv3:accessibility_metric',
         'tbmv3:cpu_time_metric',
     ])
     loading_metrics_category.AugmentOptionsForLoadingMetrics(options)