[tools/perf] Don't fail on missing proto traces

We are going to start adding first TBMv3 metrics to benchmarks and running
them on FYI bots. Because proto trace generation is still an experimental
feature and is not enabled everywhere, some bots will see TBMv3 metrics
defined but no proto traces. So we make this only a warning for the time being.

Bug: 990304
Change-Id: I9eee37e315bb8e82eae42b0ad4de8a3fcfee9942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948926Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/master@{#721033}

tools/perf/core/results_processor/compute_metrics.py

@@ -99,9 +99,12 @@ def ComputeTBMv3Metrics(test_result, trace_processor_path):
     return
 
   if CONCATENATED_PROTO_NAME not in artifacts:
-    util.SetUnexpectedFailure(test_result)
-    logging.error('%s: No proto traces to compute metrics on.',
-                  test_result['testPath'])
+    # TODO(crbug.com/990304): This is only a warning now, because proto trace
+    # generation is enabled only on selected bots. Make this an error
+    # when Telemetry is switched over to proto trace generation everywhere.
+    # Also don't forget to call util.SetUnexpectedFailure(test_result).
+    logging.warning('%s: No proto traces to compute metrics on.',
+                    test_result['testPath'])
     return
 
   for metric in metrics: