Fix millisecond calculation for millisecs_diff()

Previously, millisecs_diff() did not return milliseconds correctly, so it was not collecting according to the period settings for the profile and history.

Author: dummy

Diff for: collector.c

@@ -306,11 +306,11 @@ static int64
 millisecs_diff(TimestampTz tz1, TimestampTz tz2)
 {
 	long	secs;
-	int		millisecs;
+	int		microsecs;
 
-	TimestampDifference(tz1, tz2, &secs, &millisecs);
+	TimestampDifference(tz1, tz2, &secs, &microsecs);
 
-	return secs * 1000 + millisecs;
+	return secs * 1000 + microsecs / 1000;
 
 }