add variants of the VictoriaMetrics builders

[vrischmann]

In some cases we need to create a metric in a specific metrics.Set, for example if you want to unregister a bunch of metrics at some point the easiest way to do that is to use a dedicated set and use metrics.UnregisterSet.

This PR adds variants of the VM helpers that allow creating the metric in a specific set instead of the global set.

[wazazaby]

Hi! Thank you for your contribution.

Could you please update all the godoc to follow what I described in my comment ?

Also, looking at the metrics.Set type and it's methods, there's no nil-check meaning we will run into a NPE if a nil set is passed. I wonder if we should add a nil-check in these methods, what do you think ?

[vrischmann]

Also, looking at the metrics.Set type and it's methods, there's no nil-check meaning we will run into a NPE if a nil set is passed. I wonder if we should add a nil-check in these methods, what do you think ?

In my opinion this is a case where providing a nil set is unexpected, aka a programmer error.

If we get a nil set we only have two options:

• return a nil metric type
• panic

In the first case, it would mean that the caller will either:

• have to handle this with a nil-check, but at this point why not just nil-check the set before calling ?
• panic trying to call a method on the metric, which is nil

In the second case, there's not much point using a custom panic since it essentially has the same meaning as a runtime nil-panic.

[wazazaby]

Agreed, thanks for the analysis.

Let's keep is this way.

[wazazaby]

LGTM, thank you!