feat: expose agent stats via Prometheus endpoint (#7115)

* WIP

* WIP

* WIP

* Agents

* fix

* 1min

* fix

* WIP

* Test

* docs

* fmt

* Add timer to measure the metrics collection

* Use CachedGaugeVec

* Unit tests

* WIP

* WIP

* db: GetWorkspaceAgentStatsAndLabels

* fmt

* WIP

* gauges

* feat: collect

* fix

* fmt

* minor fixes

* Prometheus flag

* fix

* WIP

* fix tests

* WIP

* fix json

* Rx Tx bytes

* CloseFunc

* fix

* fix

* Fixes

* fix

* fix: IgnoreErrors

* Fix: Windows

* fix

* reflect.DeepEquals

docs/api/general.md

@@ -271,6 +271,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
         "host": "string",
         "port": "string"
       },
+      "collect_agent_stats": true,
       "enable": true
     },
     "provisioner": {