Adding fuzzing session duration metric (#4336)

### Motivation

We currently lack metrics for fuzzing session duration. This PR adds
that as a histogram metric, with granularity by fuzzer, job and
platform.

Part of #4271

src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py

@@ -1718,6 +1718,7 @@ def do_blackbox_fuzzing(self, fuzzer, fuzzer_directory, job_type):
 
   def run(self):
     """Run the fuzzing session."""
+    start_time = time.time()
     # Update LSAN local blacklist with global blacklist.
     global_blacklisted_functions = (
         self.uworker_input.fuzz_task_input.global_blacklisted_functions)
@@ -1871,6 +1872,14 @@ def run(self):
     self.fuzz_task_output.fuzzer_revision = self.fuzzer.revision
     self.fuzz_task_output.crash_groups.extend(crash_groups)
 
+    fuzzing_session_duration = time.time() - start_time
+    monitoring_metrics.FUZZING_SESSION_DURATION.add(
+        fuzzing_session_duration, {
+            'fuzzer': self.fuzzer_name,
+            'job': self.job_type,
+            'platform': environment.platform()
+        })
+
     return uworker_msg_pb2.Output(fuzz_task_output=self.fuzz_task_output)  # pylint: disable=no-member
 
   def postprocess(self, uworker_output):

src/clusterfuzz/_internal/metrics/monitoring_metrics.py

@@ -91,6 +91,19 @@
     ],
 )
 
+# This metric tracks fuzzer setup and data bundle update,
+# fuzzing time and the time to upload results to datastore
+FUZZING_SESSION_DURATION = monitor.CumulativeDistributionMetric(
+    'task/fuzz/session/duration',
+    bucketer=monitor.FixedWidthBucketer(width=0.05, num_finite_buckets=20),
+    description=('Total duration of fuzzing session.'),
+    field_spec=[
+        monitor.StringField('fuzzer'),
+        monitor.StringField('job'),
+        monitor.StringField('platform'),
+    ],
+)
+
 JOB_TOTAL_FUZZ_TIME = monitor.CounterMetric(
     'task/fuzz/job/total_time',
     description=('The total fuzz time in seconds '