[feature-request] Support Prometheus in Pouch about pouch

[WIZARD-CXY]

Things we need to monitor using prometheus:

• Any api duration or request count, for example: latency for pulling a image. Ideally, we will monitor all the apis we serve, but we only monitor the image and container related stuff for now.

• Any daemon operation or data we think important that we must record.

• Some go-related metrics like gc duration, #goroutines we currently have.

[WIZARD-CXY]

Get, I will take a lead for this.

[allencloud]

Do you have any experience in Prometheus?
Maybe this would be helpful to Pouch.
ping @YaoZengzeng @jintingying @zzchun 🦁

[WIZARD-CXY]

Plan to open separated issue and pr for tracking the above three area.

[allencloud]

I think this could be closed, since @WIZARD-CXY has put forward pr and docs.