Support metric labels (prometheus).

[bgrozev]

• feat: Support labels for CounterMetric.
• feat: Add labels support to DoubleGaugeMetric.
• feat: Add labels support to LongGaugeMetric.
• feat: Add labels support to BooleanMetric.
• feat: Add labels support to InfoMetric.

[JonathanLennox]

Is it really useful for the labeled and non-labeled versions of the metrics to be the same classes? They seem like two operations with disjoint functionality. I.e., if you use labels, you must not pass an initial value, you must not use get(), you must not convert to JSON, etc. Wouldn't it be easier and safer just to have separate labeled metric classes that don't have those APIs?

[bgrozev]

Is it really useful for the labeled and non-labeled versions of the metrics to be the same classes? They seem like two operations with disjoint functionality. I.e., if you use labels, you must not pass an initial value, you must not use get(), you must not convert to JSON, etc. Wouldn't it be easier and safer just to have separate labeled metric classes that don't have those APIs?

You just have to use labels consistently (match the number of labels) in the constructor and getters/setters. DoubleGaugeMetric is 143 lines in the PR, we'd have to split it in two classes of 110 (with labels) and 83 (no labels) lines. I think that's too much duplication.