feat: register the metrics with manager

[viveksyngh]

No description provided.

[prometherion]

Let's address the linters complains, and let's add // nolint:gochecknoinits and // nolint:gochecknoglobals.

[prometherion]

I know this is out of the scope of this PR, however, I got a question since you're the author of the following:
https://github.com/clastix/capsule-proxy/blob/8ed733e4b0100879be30570b0eacc45904fdb300/internal/webserver/middleware/metrics.go#L33

Why are we hard-coding the status code to be 200? This could be misleading since if I'm unauthenticated, I'm expecting 403.

curl localhost:8001/api/v1/namespaces -v                                                                                                                 (kind-dataprotector/default)
*   Trying 127.0.0.1:8001...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8001 (#0)
> GET /api/v1/namespaces HTTP/1.1
> Host: localhost:8001
> User-Agent: curl/7.68.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 403 Forbidden
< Cache-Control: no-cache, private
< Content-Length: 309
< Content-Type: application/json
< Date: Mon, 27 Jun 2022 16:11:25 GMT
< X-Content-Type-Options: nosniff
< X-Kubernetes-Pf-Flowschema-Uid: 825c0a5c-a119-4db2-bb6a-36ca1e73cc99
< X-Kubernetes-Pf-Prioritylevel-Uid: 2d66fcb3-6520-4240-9b72-8d8a7c516b13
< 
{
  "kind": "Status",
  "apiVersion": "v1",
  "metadata": {
    
  },
  "status": "Failure",
  "message": "namespaces is forbidden: User \"alice\" cannot list resource \"namespaces\" in API group \"\" at the cluster scope",
  "reason": "Forbidden",
  "details": {
    "kind": "namespaces"
  },
  "code": 403
* Connection #0 to host localhost left intact
}

Just a silly example, it could be any other error.

[viveksyngh]

@prometherion The status code is not getting hardcoded and it will always be the same as the status code returned handler.

we are creating a response writer with the below statement

https://github.com/clastix/capsule-proxy/blob/8ed733e4b0100879be30570b0eacc45904fdb300/internal/webserver/middleware/metrics.go#L57

and this response writer is getting passed to handlers in the chain which will be updating the status code to be same as what is returned by the handler
https://github.com/clastix/capsule-proxy/blob/8ed733e4b0100879be30570b0eacc45904fdb300/internal/webserver/middleware/metrics.go#L33

and we are passing the updated status code to Prometheus metrics labels, the 200 is put just to create a response writer with a valid HTTP status code.
https://github.com/clastix/capsule-proxy/blob/8ed733e4b0100879be30570b0eacc45904fdb300/internal/webserver/middleware/metrics.go#L60

[prometherion]

Thanks for the kind explanation of this PR, LGTM, and astonishing work! 🚀