Add Prometheus exporter

[reyang]

Add Prometheus exporter to prove metrics SDK. Consider adding console exporter for metrics.

[alolita]

Hi @reyang we are starting to work on building the Prometheus exporter for the metrics SDK. Can you please assign this issue to me @alolita. I will provide the handles of our interns working on this too.

[alolita]

Our interns are @CunjunWang, @erichsueh3 - they are in the process of becoming members.

[alolita]

Hi @reyang greetings! Can you please also add my intern engineers @CunjunWang, @erichsueh3 to this issue? We're working on the exporter implementation and have already submitted the design for review. Please see #172 Many thanks Reiley!

[reyang]

Hi @reyang greetings! Can you please also add my intern engineers @CunjunWang, @erichsueh3 to this issue? We're working on the exporter implementation and have already submitted the design for review. Please see #172 Many thanks Reiley!

Currently I cannot add them since their names won't show up (not a member). Happy to sponsor them to be members of OpenTelemetry.

[erichsueh3]

@reyang, can you assign me to this issue? I believe I've just been added as a member of OpenTelemetry.

[lalitb]

closing as initial implementation was completed as part of #280 #282 .
The implementation may change as metrics specs is being revisited.