Plug-in gauges into the Metric-Registry and register Metric-Registry with Me…

[mayurkale22]

…tricProducer.

The Gauges API work is split into multiple PRs.
Completed:

• Add metric registry and validations util (Add metric registry and validations util #203)
• Add Double and INT64 Gauges APIs (Add Double and INT64 Gauges APIs #207)
• Add derived gauges API (Add derived gauge API #220) - In review.

Current:

• Plug-in gauges into the registry and register Metric-Registry with MetricProducer.

Next:

• Plug-in derivedgauges into the registry (dependency with Add derived gauge API #220)
• Add MetricProducerManager to keep track of all MetricProducers i.e. MetricProducerForRegistry and MetricProducerForStats

[songy23]

LGTM

[mayurkale22]

Thanks @justindsmith @OsvaldoRosado @songy23 for reviews.