bugfix: Initialize metrics with empty tags

[Fabianoshz]

what

Added empty tags to metrics initialization to avoid errors when running commands.

Tried to hide the implementation behind the ProjectScopeTags so we can have an easy way to implement empty tags like this:

	projectTags := command.ProjectScopeTags{}
	scope = metrics.SetScopeTags(scope.SubScope("project"), projectTags.Loadtags())

Instead of this:

	scope = scope.SubScope("project")
	scope = scope.Tagged(map[string]string{
		"base_repo":         "",
		"pr_number":         "",
		"project":           "",
		"project_path":      "",
		"terraform_version": "",
		"workspace":         "",
	})

This should ensure that in the future if someone adds new tags to the ProjectScopeTags they still will be initialized empty while still can use the SetProjectScopeTags to add the values.

why

• This PR bugfix: Metrics initialization #2767 broke atlantis commands

references

• Closes potential tally.Scope() vs Prometheus usage contract mismatch #2846

[nitrocode]

Should this be named like this?

Suggested change

	func (s ProjectScopeTags) Loadtags() map[string]string {
	func (s ProjectScopeTags) loadTags() map[string]string {

[Fabianoshz]

Unfortunately I think we have to keep this public since it is used on server/events/instrumented_project_command_runner.go, maybe the function name choice wasn't clear enough?

I guess that to make this private I could make an ScopeTags interface on the metrics package, and call the loadTags from there. Then ProjectScopeTags would be an implementation of that interface passed as argument to a function that load the tags on the scope. I'm not so sure this would work though, but I can give a try.

[nitrocode]

That's ok, this was a nitpick.

[nitrocode]

Looks good. Left one nitpick.