Metrics Docs Improvements

[cijothomas]

1. Show best practices.
2. Warn users about high-cardinality, and the built-in capping.
3. Iterate that MeterProvider, Meter and Instruments should be created once and re-used.
4. Call out that creating Instrument of same name from different modules will get the same instrument instance. This maybe leveraged to avoid passing around instruments across module.
5. Show that every additional attributes increases memory/cpu cost, and recommend Resource for fully static attributes and Meter scope attributes for fixed-within-scope attributes.
6. Show proper initialization/shutdown tricks.
7. If Views make it to 1.0, show how to use views to drop unwanted attributes.
8. // More