added associated types to ensure concrete type returns for protocols

[bryce-b]

This PR will fix the issue of metric builders needing to be cast to access certain methods.

Prior to this change:

    let meterProvider = StableMeterProviderBuilder()
      .registerMetricReader(reader: myReader)
      .build()
    let meter = meterProvider.meterBuilder(name: "meter").build() as! StableMeterSdk
    let instrument = (meter.upDownCounterBuilder(name: "updown") as! LongUpDownCounterBuilderSdk)
      .setUnit("unit")
      .setDescription("description")
      .build() as! LongUpDownCounterSdk

after

let meterProvider = StableMeterProviderBuilder()
      .registerMetricReader(reader: myReader)
      .build()
    let meter = meterProvider.meterBuilder(name: "meter").build()
    let instrument = meter.upDownCounterBuilder(name: "updown")
      .setUnit("unit")
      .setDescription("description")
      .build()

One issue I have yet to solve is how to deal with the StableMeterProviderSdk. It's not possible to return the NOOP provider when no reader is registered.

  public func meterBuilder(name: String) -> MeterBuilderSdk {
//    if registeredReaders.isEmpty {
//      return DefaultStableMeterProvider.noop()
//    }

[martin308]

makes sense to me. One question about a block of commented out code

[bryce-b]

I was able to resolve the issue by adding the NoopStableMeterProviderBuilder, which is the default builder, and it will return a StableMeterProviderBuilder from its registerMetricReader builder method.