Complicated API

As a user, it is somewhat complex and quite puzzling using the API
There are a lot of components
The components are doing different things as I would expect based on their names
I think it would worth a review and refactor to make it more usable.

Examples

GlobalMetricsProvider
- Based on its name, it should provide Metrics instances
- In fact, it provides two things neither of them are Metrics
  - get returns a MeterProvider
  - getMeter returns a Meter
MeterProvider
- Based on its name, it should provide Meter instances
- Its get method does return a Meter instance
- But getDefault (means "get the default Meter") returns a MeterProvider
- Should not getDefault be getDefaultInstance?
Meter
- Its javadoc says: "Meter is a simple, interface that allows users to record measurements (metrics)."
- In fact, Meter gives you builders
- Isn't Meter something that you use to measure something?
- Like a "thermometer" measures temperature a "barometer" measures pressure, etc.
Instrument
- To me, this seems to be the component that should be called Meter
- Its javadoc is a little confusing too: "Base interface for all metrics defined in this package."
LongCounterSdk
- Based on its name, it is a special type of SDK not a special LongCounter
MetricProducer
- It has one method: Collection<MetricData> collectAllMetrics();
- Based on this, shouldn't it be called MetricCollector?
GlobalMetricsProvider and MetricProducer seems confusing next to each other
There are “missed” opportunities to use Java functional interfaces like Supplier for the Producer/Provider classes