force measurement value to type float before writing to db

whole number values (such as 0) are json decoded as type int
If the value field is first written to InfluxDB as type int (example: 0)
and later as type float (example 0.123), InfluxDB throws a
"field type conflict" exception. Forcing all data to type float fixes.

Change-Id: Ifc139bbc67077137501ffe9045a988b8afc393e4

monasca_persister/persister.py

@@ -526,7 +526,7 @@ class MetricPersister(AbstractPersister):
         time_stamp = metric['timestamp']
         LOG.debug('timestamp %s', time_stamp)
 
-        value = metric['value']
+        value = float(metric['value'])
         LOG.debug('value: %s', value)
 
         if 'value_meta' in metric and metric['value_meta']: