Make the data gather singleton support multiple instances

Due to we use pgbouncer, we could share the same instance of the metrics
gatherer singleton in multiples requests. Added support to have
multiple instances identified by transaction id

server/lib/python/cartodb_services/cartodb_services/metrics/log.py

@@ -1,9 +1,10 @@
-from datetime import datetime
 import abc
 import json
 import re
 import time
 import uuid
+import plpy
+from datetime import datetime
 from contextlib import contextmanager
 from urlparse import urlparse
 
@@ -78,7 +79,9 @@ class MetricsDataGatherer:
         def clean(self):
             self.data = {}
 
-    __instance = None
+
+    #  We use pgbouncer so we need to have multiples instances per request id
+    __instance = {}
 
     @classmethod
     def add(self, key, value):
@@ -98,10 +101,16 @@ class MetricsDataGatherer:
 
     @classmethod
     def instance(self):
-        if not MetricsDataGatherer.__instance:
-            MetricsDataGatherer.__instance = MetricsDataGatherer.__MetricsDataGatherer()
+        txid = MetricsDataGatherer._get_txid()
+        if txid not in MetricsDataGatherer.__instance:
+            MetricsDataGatherer.__instance[txid] = MetricsDataGatherer.__MetricsDataGatherer()
 
-        return MetricsDataGatherer.__instance
+        return MetricsDataGatherer.__instance[txid]
+
+    @classmethod
+    def _get_txid(self):
+        result = plpy.execute('select txid_current() as txid')
+        return result[0]['txid']
 
 
 class MetricsServiceLoggerFactory:
@@ -168,7 +177,6 @@ class MetricsLogger(object):
             json.dump(r, log_file)
             log_file.write('\n')
 
-
     @property
     def service_config(self):
         return self._service_config
@@ -237,9 +245,7 @@ class MetricsIsolinesLogger(MetricsLogger):
 
     def collect_data(self, data):
         dump_data = super(MetricsIsolinesLogger, self).collect_data(data)
-
         dump_data.update({
             "isolines_generated": data.get('isolines_generated', 0)
         })
-
         return dump_data