Add an option to expose Prometheus metrics via http/s server

[bszirtes]

Description

Initialized Prometheus metric types and expanded current metrics to push values into Prometheus too, if enabled.

Issue link

networkservicemesh/sdk#1652

How Has This Been Tested?

• Added unit testing to cover
• Tested manually
• Tested by integration testing
• Have not tested

Types of changes

• Bug fix
• New functionality
• Documentation
• Refactoring
• CI

[denis-tingaikin]

LGTM.

Could you fix linter?

[bszirtes]

Hi @denis-tingaikin,

Thank you for the feedback! Regarding the linter issue, do you have any recommendations on where to initialize the Prometheus metric types? I'm considering moving the content of my init function (since that's causing the linting issue) into a new function and calling it directly from the forwarder. Would that be a better approach?

[denis-tingaikin]

I think we can define static sync.Once in the stats/prometheus and do initialization in the NewServer function if promeheus enabled.

[bszirtes]

May I ask which specific NewServer function you are referring to @denis-tingaikin?

[denis-tingaikin]

Sure, I'd try something like this:

in the prometheus.go

package stats

+ import "sync"

+ var prometheusInitOnce sync.Once

-  func init() {
+ func registerMetrics() {
+   if !prometheus.Enabled() {
+     return  
+   }
  ...
}

Then we could initiate it in the stats client/server https://github.com/networkservicemesh/sdk-vpp/blob/main/pkg/networkservice/metrics/stats/server.go#L44, https://github.com/networkservicemesh/sdk-vpp/blob/main/pkg/networkservice/metrics/stats/client.go#L46

via adding one line

prometheusInitOnce.Do(registerMetrics)

[bszirtes]

Hi @denis-tingaikin,

Thank you for the clarification! I've made the changes as you recommended.