Add support for gauge metric in static-exporter

[gauravgpta93]

Add support for gauge metric in static-exporter
Keep default as counter to avoid any changes for other places

[jdbaldry]

nit: for consistency with the other withFUNCTION methods, perhaps it should be just withType and not withMetricType?

[Duologic]

You could set this value below and require withMetricType instead of overloading the constructor.

[jdbaldry]

Is there a reason to prefer that over having all the arguments in the constructor?
I'm not sure I see how metricType is different from description or name except that the library originally only supported counter types.

In other words, why have:

metric.new('NAME', 'DESCRIPTION') + metric.withMetricType('TYPE')

when you can just have:

metric.new('NAME', 'DESCRIPTION, 'TYPE')

[jdbaldry]

Also, to help me understand our patterns better. Why does the constructor do:

self.withName(name)
+ self.withDescription(description)
+ self.withMetricType(metricType)

instead of:

{
  name: name,
  description: description,
  type: metricType,
}

I am thinking it's to use the public API presented by the object so that if withName ever does validation, the constructor also does the validation without refactor?
In that case, should we have some convention of indicating that the type, description, and type fields shouldn't be modified except through that public API. Perhaps they should be fields within a hidden _internal field or something?