Feature request: include collection of SSL expiration from SHOW GLOBAL STATUS output

[nikosmeds]

Issue

It appears that mysqld-exporter does not collect any non-floating point values from SHOW GLOBAL STATUS;

1. Use SQL query search GLOBAL STATUS

   mysql> SHOW GLOBAL STATUS \G;
   [...]
   *************************** 389. row ***************************
   Variable_name: Ssl_finished_connects
           Value: 0
   *************************** 390. row ***************************
   Variable_name: Ssl_server_not_after
           Value: May 24 11:46:23 2020 GMT
   *************************** 391. row ***************************
   Variable_name: Ssl_server_not_before
           Value: Feb 24 11:46:23 2020 GMT
   *************************** 392. row ***************************
   Variable_name: Ssl_session_cache_hits
           Value: 0
   

2. Via PromQL I can find mysql_global_status_ssl_finished_connects and mysql_global_status_ssl_session_cache_hits metrics
3. Via PromQL I cannot find mysql_global_status_ssl_server_not_after or mysql_global_status_ssl_server_not_before metrics

My assumption is that these metrics are skipped due to the SQL command output format being timestamps and not a floating point value.

It would be great to have the SSL expiration metrics available in Prometheus, as then I can create "SSL Expiring Soon" alerts for the certificates loaded by MariaDB.

Thanks :)

Host operating system:

Linux redacted.example.net 5.0.0-36-generic #39~18.04.1-Ubuntu SMP Tue Nov 12 11:09:50 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

mysqld_exporter version:

mysqld_exporter, version 0.12.1 (branch: HEAD, revision: 48667bf7c3b438b5e93b259f3d17b70a7c9aff96)
  build user:       root@0b3e56a7bc0a
  build date:       20190729-12:35:58
  go version:       go1.12.7

MySQL server version

Percona XtraDB (i.e. MariaDB)

mysql> SELECT VERSION();
+------------------+
| VERSION()        |
+------------------+
| 5.7.28-31-57-log |
+------------------+
1 row in set (0.00 sec)

[nikosmeds]

I have minimal Go experience, but #338 looks like a good example of the changes required when adding new metrics that aren't initially floating point values.

Also blackbox-exporter collects SSL expiration with HTTP requests, https://github.com/prometheus/blackbox_exporter/blob/master/prober/http.go#L260 may be another good example.

EDIT: Also related https://programming.guide/go/format-parse-string-time-date-example.html

[nikosmeds]

Figured I'd update with a recent discovery: we can covert the value to epoch time using date command.

$ date -d "May 24 11:46:23 2020 GMT" +'%s'
1590320783