[benchmarking] add trace for add_reserved_diagnostics cmd call

Add a trace for the call to add_reserved_diagnostics to help debug.

Bug: chromium:1133791
Change-Id: I2a94feda38d6de9f7be7992dacde064e2d1458b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441199
Auto-Submit: Wenbin Zhang <wenbinzhang@google.com>
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812400}

tools/perf/core/results_dashboard.py

@@ -173,6 +173,7 @@ def MakeHistogramSetWithDiagnostics(histograms_file,
   output_path = os.path.join(output_dir, test_name + '.json')
   cmd = ([sys.executable, add_reserved_diagnostics_path] +
          add_diagnostics_args + ['--output_path', output_path])
+  logging.info(cmd)
   subprocess.check_call(cmd)