Metrics: create independent MeterProvider

[jmacd]

The existing Metrics SDK supports a MeterProvider API but the underlying components treat the Resource as part of the export Record and do not formally recognize a division between Meters. This issue outlines a proposal to solve this. The outcome will be that each:

1. metric.Descriptor no longer includes instrumentation name and version, nor Resource
2. metric/registry supports a UniqueMeterProvider that maintains a map of named MeterImpls.
3. metrics Export() includes additional SourceData (instrumentation name and version, schema URL, Resource)
4. sdk/metric/controller maintains once accumulator/checkpointer per Meter, exports them separately
5. OTLP exporter for metrics no longer needs to group data, is much simpler.

This is too much change for a single PR, but I have prototyped it in #2085. This issue will cover breaking up the draft PR into smaller pieces.

Another goal is met in the draft PR above, that metric.Descriptor could be moved out of the top-level metric package into a sub-package (possibly named "sdkapi"), as demonstrated in #2044.

[jmacd]

Discussed in the Go SIG meeting. The main objection to the draft PR #2085 is that it splits Metrics exports into one call per Meter. Instead, follow what's been done for Trace, export a slice of InstrumentationLibraryMetrics.

[jmacd]

Next steps: I will continue with the refactoring proposed here in smaller PRs.

1. Remove instrumentation library and version from metric.Descriptor. Pass this information through a MeterProvider and store it in the metric Controller. Add registry.UniqueMeterProvider.
2. Create a metric/sdkapi subpackage and move Descriptor, InstrumentImpl, MeterImpl, SyncImpl, AsyncImpl, and a few other implementation-specific types into this package. These are the types that will survive regardless of which API design is chosen, as they support the current SDK's low-level interface shared by any API.
3. Remove Resource from the metric export Record.
4. Remove grouping from OTLP exporter, receive a slice of InstrumetationLibraryMetric recods.