Detect SIReadLock locks in the pg_locks metric (prometheus-community#421)

Co-authored-by: Will Rouesnel <wrouesnel@wrouesnel.com>

Author: ocharles

cmd/postgres_exporter/postgres_exporter.go

@@ -379,7 +379,8 @@ var queryOverrides = map[string][]OverrideQuery{
  ('sharelock'),
  ('sharerowexclusivelock'),
  ('exclusivelock'),
- ('accessexclusivelock')
+ ('accessexclusivelock'),
+ ('sireadlock')
 ) AS tmp(mode) CROSS JOIN pg_database
 LEFT JOIN
  (SELECT database, lower(mode) AS mode,count(*) AS count