Add Fuzzer.created_at field

We want to start tracking more metrics around blackbox fuzzers. We
currently only track the last updated timestamp of a fuzzer. This
introduces a craeted_at timestamp which is set when creating a fuzzer.

Testing:
Ran a local server and creatd a Fuzzer to verify the field is set.

Author: dylanjew

src/appengine/handlers/fuzzers.py

@@ -251,6 +251,8 @@ def post(self):
     fuzzer = data_types.Fuzzer()
     fuzzer.name = name
     fuzzer.revision = 0
+    fuzzer.created_at = datetime.datetime.now(tz=datetime.timezone.utc).replace(
+        tzinfo=None)
     return self.apply_fuzzer_changes(fuzzer, upload_info)
 
 

src/clusterfuzz/_internal/datastore/data_types.py

@@ -261,6 +261,10 @@ class Blacklist(Model):
 class Fuzzer(Model):
   """Represents a fuzzer."""
 
+  # Created at timestamp. Not set for fuzzers created before this field was
+  # added.
+  created_at = ndb.DateTimeProperty()
+
   # Additionally allows '.' and '@' over NAME_CHECK_REGEX.
   VALID_NAME_REGEX = re.compile(r'^[a-zA-Z0-9_@.-]+$')