hydra-notify: support sending diagnostic dumps to STDERR on request

Dependencies

• [2] 4H6FVIWG hydra-notify: make the prometheus endpoint configurable, default-off
• [*] FUOOHCAA Init a docs section for monitoring, document queue-runner-status and the prometheus metrics
• [*] IE2PRAQU hydra-queue-runner: Send build notifications
• [*] P4SME2BC Abstract over postgres' LISTEN/NOTIFY
• [*] FN3QFV6V hydra-notify: Create a helper for running each plugin on an event
• [*] I7DFJWL6 hydra-notify: initial scratch take of prometheus events

Change contents

• edit in doc/manual/src/monitoring/README.md at line 24
  [2.766]
  +

  + ### Diagnostic Dump

  +

  + The notification daemon can also dump its metrics to stderr whether or not the exporter

  + is configured. This is particularly useful for cases where metrics data is needed but the

  + exporter was not enabled.

  +

  + To trigger this diagnostic dump, send a Postgres notification with the

  + `hydra_notify_dump_metrics` channel and no payload. See

  + [Re-sending a notification](../notifications.md#re-sending-a-notification).
• edit in src/script/hydra-notify at line 102
  [6.1922]

  [7.0]
  + $listener->subscribe("hydra_notify_dump_metrics");
• edit in src/script/hydra-notify at line 148
  [8.1170]

  [8.1170]
  +

  + if ($channelName eq "hydra_notify_dump_metrics") {

  + print STDERR "Dumping prometheus metrics:\n${\$prom->format}\n";

  + next;

  + }