docs: Add section about monitoring with Prometheus

This patch adds a section to the monitoring docs with an example of how
to use the prometheus-expvar-exporter to monitor chasquid with
Prometheus.

docs/monitoring.md

@@ -92,3 +92,114 @@ List of exported variables:
   errors as part of keeping the STS cache.
 - **chasquid/version** (string): version string.
 
+
+## Prometheus
+
+To monitor chasquid using [Prometheus](https://prometheus.io), you can use the
+[prometheus-expvar-exporter](https://blitiri.com.ar/git/r/prometheus-expvar-exporter/b/master/t/f=README.md.html)
+with the following configuration:
+
+```toml
+# Address to listen on. Prometheus should be told to scrape this.
+listen_addr = ":8000"
+
+[chasquid]
+# Replace with the address of chasquid's monitoring server.
+url = "http://localhost:1099/debug/vars"
+
+# Metrics are auto-imported, but some can't be; in particular the ones with
+# labels need explicit definitions here.
+
+m.aliases_hook_results.expvar ="chasquid/aliases/hookResults"
+m.aliases_hook_results.help ="aliases hook results"
+m.aliases_hook_results.label_name ="result"
+
+m.deliver_attempts.expvar = "chasquid/queue/deliverAttempts"
+m.deliver_attempts.help = "attempts to deliver mail"
+m.deliver_attempts.label_name = "recipient_type"
+
+m.dsn_queued.expvar = "chasquid/queue/dsnQueued"
+m.dsn_queued.help = "DSN queued"
+
+m.items_written.expvar = "chasquid/queue/itemsWritten"
+m.items_written.help = "items written"
+
+m.queue_puts.expvar = "chasquid/queue/putCount"
+m.queue_puts.help = "chasquid/queue/putCount"
+
+m.smtpin_commands.expvar = "chasquid/smtpIn/commandCount"
+m.smtpin_commands.help = "incoming SMTP command count"
+m.smtpin_commands.label_name = "command"
+
+m.smtp_hook_results.expvar = "chasquid/smtpIn/hookResults"
+m.smtp_hook_results.help = "hook invocation results"
+m.smtp_hook_results.label_name = "result"
+
+m.loops_detected.expvar = "chasquid/smtpIn/loopsDetected"
+m.loops_detected.help = "loops detected"
+
+m.smtp_response_codes.expvar = "chasquid/smtpIn/responseCodeCount"
+m.smtp_response_codes.help = "response codes returned to SMTP commands"
+m.smtp_response_codes.label_name = "code"
+
+m.in_sec_level_checks.expvar = "chasquid/smtpIn/securityLevelChecks"
+m.in_sec_level_checks.help = "incoming security level check results"
+m.in_sec_level_checks.label_name = "result"
+
+m.spf_results.expvar = "chasquid/smtpIn/spfResultCount"
+m.spf_results.help = "SPF result count"
+m.spf_results.label_name = "result"
+
+m.in_tls_usage.expvar = "chasquid/smtpIn/tlsCount"
+m.in_tls_usage.help = "count of TLS usage in incoming connections"
+m.in_tls_usage.label_name = "status"
+
+m.out_sec_level_checks.expvar = "chasquid/smtpOut/securityLevelChecks"
+m.out_sec_level_checks.help = "outgoing security level check results"
+m.out_sec_level_checks.label_name = "result"
+
+m.sts_modes.expvar = "chasquid/smtpOut/sts/mode"
+m.sts_modes.help = "STS checks on outgoing connections, by mode"
+m.sts_modes.label_name = "mode"
+
+m.sts_security.expvar = "chasquid/smtpOut/sts/security"
+m.sts_security.help = "STS security checks on outgoing connections, by result"
+m.sts_security.label_name = "result"
+
+m.out_tls_usage.expvar = "chasquid/smtpOut/tlsCount"
+m.out_tls_usage.help = "count of TLS usage in outgoing connections"
+m.out_tls_usage.label_name = "status"
+
+m.sts_cache_expired.expvar = "chasquid/sts/cache/expired"
+m.sts_cache_expired.help = "expired entries in the STS cache"
+
+m.sts_cache_failed_fetch.expvar = "chasquid/sts/cache/failedFetch"
+m.sts_cache_failed_fetch.help = "failed fetches in the STS cache"
+
+m.sts_cache_fetches.expvar = "chasquid/sts/cache/fetches"
+m.sts_cache_fetches.help = "total fetches in the STS cache"
+
+m.sts_cache_hits.expvar = "chasquid/sts/cache/hits"
+m.sts_cache_hits.help = "hits in the STS cache"
+
+m.sts_cache_invalid.expvar = "chasquid/sts/cache/invalid"
+m.sts_cache_invalid.help = "invalid policies in the STS cache"
+
+m.sts_cache_io_errors.expvar = "chasquid/sts/cache/ioErrors"
+m.sts_cache_io_errors.help = "I/O errors when maintaining STS cache"
+
+m.sts_cache_marshal_errors.expvar = "chasquid/sts/cache/marshalErrors"
+m.sts_cache_marshal_errors.help = "marshalling errors when maintaining STS cache"
+
+m.sts_cache_refresh_cycles.expvar = "chasquid/sts/cache/refreshCycles"
+m.sts_cache_refresh_cycles.help = "STS cache refresh cycles"
+
+m.sts_cache_refresh_errors.expvar = "chasquid/sts/cache/refreshErrors"
+m.sts_cache_refresh_errors.help = "STS cache refresh errors"
+
+m.sts_cache_refreshes.expvar = "chasquid/sts/cache/refreshes"
+m.sts_cache_refreshes.help = "count of STS cache refreshes"
+
+m.sts_cache_unmarshal_errors.expvar = "chasquid/sts/cache/unmarshalErrors"
+m.sts_cache_unmarshal_errors.help = "unmarshalling errors in STS cache"
+```