Protect call to pg_current_wal_lsn() in retained_wal query

We can't call pg_current_wal_lsn() if we are a standby instance (read
replica). Any attempt to call this function while in recovery results
in:

ERROR:  recovery is in progress

Signed-off-by: Tristan Partin <tristan@neon.tech>

compute/etc/sql_exporter/retained_wal.sql

@@ -1,5 +1,10 @@
 SELECT
   slot_name,
-  pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)::FLOAT8 AS retained_wal
+  pg_wal_lsn_diff(
+    CASE
+      WHEN pg_is_in_recovery() THEN pg_last_wal_replay_lsn()
+      ELSE pg_current_wal_lsn()
+    END,
+    restart_lsn)::FLOAT8 AS retained_wal
 FROM pg_replication_slots
 WHERE active = false;