Implement MetricReader

[dyladan]

The MetricReader handles collecting metrics from all SDK instruments. It also handles shutdown and forceflush signals from the SDK.

Methods:

• Collect
• Shutdown

Required metric readers:

• Periodic exporting metric reader
  • Collects metrics on a time interval and passes them to a push metric exporter
  • Configurations
    • exporter
    • export interval (ms)
    • export timeout (ms)

[pichlermarc]

I'd be happy to pick this one up if no one is working on it already.

[legendecas]

@pichlermarc Please go ahead! Also please take a look at the design doc of the SDK internals https://docs.google.com/document/d/1EluDhj5UanNATS3hR8xx30sv55Se7uYjsGYslvNWHtc/edit# and feel free to comment if you have any questions or ideas.