FEATURE: Additional Prometheus metrics

[tillprochaska]

Is your feature request related to a problem? Please describe.
Based on some real-world use cases, it would be great to have Prometheus metrics for the following:

• Total number of search alerts (gauge)
• Search alerts triggered (i.e. whenever a notification is sent because there is a new result matching an alert) (counter)
• Exports created (counter)
• Emails sent (counter)
• OpenRefine Reconciliation usage
• Search requests (i.e. requests to the /entities endpoint with a non-empty q URL search param; counter)
• Total number of users with a non-expired API key (gauge)
• Total number of user groups (gauge)
• Total number of entity mappings (gauge)
• Entity mappings created (counter)
• Entity mappings executed/triggered (counter)
• Total number of files (counter)

Describe the solution you'd like
-/-

Describe alternatives you've considered
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Additional context
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