Update instruments to v0.4.0 spec

[c24t]

See also open-telemetry/opentelemetry-go#729.

The v0.4.0 Metric API defines six named metric instruments that replace Measure, Counter, and Observer. Update our API to match the spec.

[AndrewAXue]

I can take this (after trace exporter is done)

[lzchen]

I can work with you on this if you really want to take it. @AndrewAXue

[AndrewAXue]

I was talking to my team and we decided that building the metrics exporter is higher priority at the time for me (its blocking the other intern's work) so I'm gonna do that first. I'd be happy to work with you on this afterwards (if its not done yet) :)

[aabmass]

@lzchen is any work being done on this? I can start on this after #664 :)

[lzchen]

I'll take this issue and create sub-issues to divide up the work :)

[aabmass]

@lzchen any preference for one of these instruments I can get started on?

[lzchen]

No preference.