fix: WAL health panel false positive on idle databases

The WAL health stat panel (ID 612) shows "Unsynced" (red) on idle
databases where no WAL activity is occurring, even though the archiver
is functioning correctly.

PostgreSQL's archive_timeout only forces WAL segment switches when
there are records in the current segment. On idle databases (common in
staging/dev), no new WAL segments are produced, causing
seconds_since_last_archival to grow indefinitely and the panel to
turn red.

Fix: multiply the archival age by whether there are actually WAL
files pending archival (ready > 0). When no WALs are pending, the
result is 0 (Healthy) since the archiver has nothing to do.

Fixes #55

Signed-off-by: Christoph Ebeling <c.ebeling@nexode.de>

Author: christophebeling

charts/cluster/grafana-dashboard.json

@@ -1973,7 +1973,7 @@
         "type": "prometheus",
         "uid": "${DS_PROMETHEUS}"
       },
-      "description": "WAL is considered Healthy when the last WAL is 0min to 6min old, Delayed when it is less than 15min and Unsynced for >15min.",
+      "description": "WAL is considered Healthy when the last WAL is 0min to 6min old, Delayed when it is less than 15min and Unsynced for >15min. Idle databases with no pending WAL files are shown as Healthy.",
       "fieldConfig": {
         "defaults": {
           "color": {
@@ -2133,7 +2133,7 @@
             "uid": "${DS_PROMETHEUS}"
           },
           "editorMode": "code",
-          "expr": "max((1 - cnpg_pg_replication_in_recovery{namespace=~\"$namespace\", pod=~\"$instances\"}) * (time() - timestamp(cnpg_pg_stat_archiver_seconds_since_last_archival{namespace=~\"$namespace\", pod=~\"$instances\"}) +\ncnpg_pg_stat_archiver_seconds_since_last_archival{namespace=~\"$namespace\", pod=~\"$instances\"}))",
+          "expr": "max((1 - cnpg_pg_replication_in_recovery{namespace=~\"$namespace\", pod=~\"$instances\"}) * (time() - timestamp(cnpg_pg_stat_archiver_seconds_since_last_archival{namespace=~\"$namespace\", pod=~\"$instances\"}) +\ncnpg_pg_stat_archiver_seconds_since_last_archival{namespace=~\"$namespace\", pod=~\"$instances\"})\n* on(pod, namespace) group_left()\n(cnpg_collector_pg_wal_archive_status{namespace=~\"$namespace\", pod=~\"$instances\", value=\"ready\"} > bool 0))",
           "hide": false,
           "instant": false,
           "legendFormat": "WAL",