sheetsync: Clear event counts before each update

Otherwise, no count of 0 ever gets set, and things are left showing
values when they shouldn't.

sheetsync/setup.py

@@ -7,7 +7,7 @@ setup(
 	install_requires = [
 		"argh",
 		"gevent",
-		"prometheus-client",
+		"prometheus-client==0.7.1", # locked version as we rely on internals
 		"psycogreen",
 		"psycopg2",
 		"python-dateutil",

sheetsync/sheetsync/main.py

@@ -201,6 +201,9 @@ class SheetSync(object):
 		for row in result.fetchall():
 			by_id[row.id] = row
 			counts[row.sheet_name, row.category, str(row.poster_moment), row.state, str(bool(row.error))] += 1
+		# Reach into metric internals and forget about all previous values,
+		# or else any values we don't update will remain as a stale count.
+		event_counts._metrics.clear()
 		for labels, count in counts.items():
 			event_counts.labels(*labels).set(count)
 		return by_id