postgres integration: fix wrong timestamp

The timestamp derived from the last `state_change` was wrong because it didn't take into account the timezone, causing some events to be dropped during intake. Now we do an explicit conversion to a unix timestamp from the UTC Unix epoch date.

postgres/datadog_checks/postgres/statement_samples.py

@@ -1,3 +1,4 @@
+import datetime
 import logging
 import os
 import re
@@ -22,6 +23,8 @@
 
 VALID_EXPLAIN_STATEMENTS = frozenset({'select', 'table', 'delete', 'insert', 'replace', 'update'})
 
+unix_epoch = datetime.datetime(1970, 1, 1, tzinfo=datetime.timezone.utc)
+
 # columns from pg_stat_activity which correspond to attributes common to all databases and are therefore stored in
 # under other standard keys
 pg_stat_activity_sample_exclude_keys = {
@@ -343,12 +346,11 @@ def _collect_plan_for_statement(self, row):
                 },
                 'postgres': {k: v for k, v in row.items() if k not in pg_stat_activity_sample_exclude_keys},
             }
+            event['timestamp'] = time.time() * 1000
             if row['state'] in {'idle', 'idle in transaction'}:
                 if row['state_change'] and row['query_start']:
                     event['duration'] = (row['state_change'] - row['query_start']).total_seconds() * 1e9
-                    event['timestamp'] = time.mktime(row['state_change'].timetuple()) * 1000
-            else:
-                event['timestamp'] = time.time() * 1000
+                    event['timestamp'] = (row['state_change'] - unix_epoch).total_seconds() * 1000
             return event
 
     def _explain_pg_stat_activity(self, rows):