Add Clojure wrapper APIs for OpenTelemetry Metrics

[cguckes]

@steffan-westcott, is this ok to merge? I'd be very interested in that feature.

[steffan-westcott]

I appreciate your interest in adding support for the Metrics API. Please note that metrics API support is part of the work in progress for the next release of clj-otel. I had mainly settled on the design by the time this pull request appeared. I apologise I have not responded earlier.

I will not be merging this pull request, as metrics support has been added since the request was raised. There are also some aspects of the chosen design I consider preferable:

• Uses the same data-oriented design as for the support of the Traces API. You can see this most clearly in the function steffan-westcott.clj-otel.api.metrics.instrument/instrument. This API of 1 function is responsible for creating an instrument specified by the map parameter it takes. Compare this to the API of 12 functions sharing the same responsibility in this pull request.
• No reflection occurs at runtime. Protocols Counter and Histogram are used to select the appropriate implementation when updating instruments.
• Complete coverage of Observable measurements.
• Support for context when updating instruments.