Docs for Metrics - add basic and advanced examples

[cijothomas]

Opening an issue to track adding docs for Metrics.

Proposing to add 2 examples to begin with:

1. Basic instrumentation - This will use stdout exporter, and show example usage of all instrument kinds. More geared towards "learning the various OTel instruments".
2. Advanced configuration - This will use OTLP Exporter, and shows advanced configuration - Views (and its various use cases), changing Temporality.

The docs alone might not be sufficient, hence the proposal to have 2 examples. Happy to iterate based on feedback.

Note: It was discussed in today's SIG meeting as well, in the context of filling the spec compliant matrix. The examples will demonstrate most common use cases, and not every line item from the spec-compliance matrix.

Related issue : #914