Remove unnecessary call to getConventionName in PrometheusMeterRegistry

[davidmestr]

We need some customization in the collector name, but in PrometheusMeterRegistry and MicrometerCollector, the collector name is calculated two times (and the second is difficult to customize):

• In PrometheusMeterRegistry (applyToCollector method) the collector name is retrieved from getConventionName(Meter.Id id) method, in order to be used as a key in the internal collectionMap.
• But, when the MicrometerCollector is created, the collector name is calculated another time in the MicrometerCollector constructor.

We propose to unify the collector name passing the first calculated name (in PrometheusMeterRegistry.getConventionName(id)) to MicrometerCollector constructor.

With this easy change, it is posible to uncouple the metric name to the collector name, and resolve some existent issues related to some Prometheus metric name restrictions that doesn't exist in Micrometer.

We also provide a PR with the small changes in the PrometheusMeterRegsitry.applyToCollector and MicrometerCollector classes.