Support location zones and resolver metrics in go client

[bvighnesha]

Proposed changes

Support the newly added per location and resolver metrics.

APIVersion upgrade 4 -> 5
NGINX Plus version upgrade R18 -> R19

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[Dean-Coakley]

Client updates looks pretty good.

Tests need to be completed before merging.

Please add a note in the PR description regarding:

• APIVersion upgrade 4 -> 5
• NGINX Plus version upgrade R18 -> R19

[pleshakov]

@b-v-r please see my comments

[Dean-Coakley]

Looks pretty good now. Just unit tests need improvement

[Rulox]

Looks good now 👍

[Dean-Coakley]

I think the resolvers test could be extended to validate metrics are correctly marshalled and returned.

Something like:

if resolver, ok := stats.Resolvers[resolverMetric]; ok {
    if resolver.Requests.Name < 1 {
        t.Errorf("Resolvers stats missing: %v", resolver.Requests)
    }
} else {
    t.Errorf("Resolver %v not found", resolverMetric)
}

The current test of configuring one resolver and testing for the existence/absence of that resolver could be considered sufficient.

I defer to your judgement. LGTM.