Allow empty namespace, namespace chain for nested structs

[burik666]

The namespace is optional.

Nested structs without a namespace are useful for embedded structs.

var metrics struct {
	BaseMetrics
	Sub         SubMetrics `namespace:"sub"`
	SomeCounter func(...) prometheus.Counter
}

[burik666]

@carrascoacd @juananrey, review it please.

[carrascoacd]

It looks good to me. Thanks for the contribution!

[yaroslavcev]

The approach LGTM, thought I'd keep behavior of public Init function as it is. Please also update the CHANGELOG.md

[yaroslavcev]

WDYT if we write whole struct branch like this:

			newNamespaces := namespaces
			namespaceTag, ok := fieldType.Tag.Lookup("namespace")
			if ok && namespaceTag != "" {
				newNamespaces = append(newNamespaces, namespaceTag)
			}
			if err := in.initMetrics(field, newNamespaces...); err != nil {
				return err
			}

[yaroslavcev]

These changes are not necessary to make namespace optional for nested structs, right?

Also. This is kind of breaking change. Before the change call to Init with empty namespace was producing _blah_blah metric (note the starting underscore). Even though it was not intentional, I'd keep this behaviour

[burik666]

Underscores are added only for nested structures.
For example:

type metrics struct {
	AAA func() prometheus.Counter `name:"aaa" help:""`
	Base struct {
		CCC func() prometheus.Counter `name:"ccc" help:""`
	}
	Sub struct {
		BBB func() prometheus.Counter `name:"bbb" help:""`
	} `namespace:"sub"`
}

This will produce aaa, ccc, and _sub_bbb metrics. This looks a bit strange.

How about replacing the namespace argument with namespaces ...string in the Init function?
This would allow metrics to be created without a prefix at all while preserving this behavior.