Add documentation for Metrics API instruments

[kaylareopelle]

The API is missing documentation for the various OpenTelemetry::Metrics::Meter#create_* instrument methods. Using YARD syntax, add documentation for these methods. You can use the Metrics API specification for information. A great example of API documentation is available for OpenTelemetry::Trace::Tracer#in_span.

• Counter
• Histogram
• Up Down Counter
• Observable Counter (spec'd as Asynchronous Counter)
• Observable Gauge (spec'd as Asynchronous Gauge)
• Observable Up Down Counter (spec'd as Asynchronous Up Down Counter)

[github-actions]

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