Export activerecord metrics via prometheus

- An existing collector already exsists we just need to start it
- Exported metrics are:
| `active_record_connection_pool_connections` | Total connections in pool |
| `active_record_connection_pool_busy`        | Connections currently in use |
| `active_record_connection_pool_dead`        | Dead connections |
| `active_record_connection_pool_idle`        | Idle connections |
| `active_record_connection_pool_waiting`     | Requests waiting for a connection |

Author: TheOneFromNorway

config/initializers/falcon.rb

@@ -3,4 +3,5 @@
 if ENV["EXPORT_WEB_METRICS"] == "true"
   require "prometheus_exporter/instrumentation"
   PrometheusExporter::Instrumentation::Process.start(type: "web")
+  PrometheusExporter::Instrumentation::ActiveRecord.start
 end

config/initializers/sidekiq.rb

@@ -47,6 +47,7 @@ def call(_worker, job, _queue)
 
     config.on :startup do
       PrometheusExporter::Instrumentation::Process.start type: "sidekiq"
+      PrometheusExporter::Instrumentation::ActiveRecord.start
       PrometheusExporter::Instrumentation::SidekiqProcess.start
       PrometheusExporter::Instrumentation::SidekiqQueue.start
       PrometheusExporter::Instrumentation::SidekiqStats.start