Initialize metrics with labels to 0

[robrichard]

In #139 metrics without labels are initialized to 0 when created. Would it be possible to have a function that can initialize metrics with labels to 0 for a known set of label values? Something like:

const summary = new promClient.Summary({
    name: 'http_request_duration_seconds',
    labelNames: labels
});
summary.init({"response_status": "success"});
summary.init({"response_status": "error"});

[jdmarshall]

Would it not make sense to do something like the following?

const summary = new promClient.Summary({
    name: 'http_request_duration_seconds',
    labelNames: labels
});

summary.observe({"response_status": "success"}, 0);
summary.observe({"response_status": "error"}, 0);

I feel as if maybe there's a more concise way to represent this, but I'd have to hear from you and perhaps a couple other people how exactly you mean to use this. How many label sets are we talking about initializing? And are these also the same values that might go with default labels?

Because if having default labels zeroed out during initialization is a useful feature, there's another story asking for that, and it might make it more compelling to have both rather than one or the other.

As it stands there are a couple of old PRs that have pretty large merge conflicts and would need to be resubmitted or have extensive surgery to get them to merge.