Metrics semconv should include value type of instrument and requirement levels in yaml and md

[lmolkova]

Metric-specific properties (in yaml) are limited to metric_name, instrument, and unit:

We never document the value type (long or float) in yaml or markdown files.

As a result code generation cannot really generate metric definitions and we leave a wiggle room for instrumentation to decide on (and change) the value type.

Also, we manually include requirement level into md.

We should

• extend metric yaml schema to include
  • value type (and make it a part of stability guarantees)
  • requirement level
• update existing metric definitions in the spec

[lmolkova]

related to #551, https://github.com/open-telemetry/oteps/pull/243

[joaopgrassi]

Also related to https://github.com/open-telemetry/semantic-conventions/issues/1040