Support a single Resource per MeterProvider in the SDK

[jmacd]

The specification for Resource SDK says that there can be just one Resource used by an individual MeterProvider (and TraceProvider). This implies we can simplify the export path to use a single Resource per Export(), which simplifies the OTLP Exporter substantially.

What changes:

• export.Record no longer contains a *resource.Resource
• Export() methods now receive a single *resource.Resource
• sdk/metric/controller/basic now passes the *resource.Resource to Export() directly
• sdk/metric/sdk.go (the Accumulator) no longer touches a *resource.Resource
• the exporters/otlp/otlpmetric code path no longer groups by *resource.Resource

Part of #2086.
Part of #2119.

[codecov]

Codecov Report

Merging #2120 (55b6db5) into main (a8bb0bf) will decrease coverage by 0.0%.
The diff coverage is 82.6%.

@@           Coverage Diff           @@
##            main   #2120     +/-   ##
=======================================
- Coverage   72.8%   72.7%   -0.1%     
=======================================
  Files        177     177             
  Lines      12148   12126     -22     
=======================================
- Hits        8848    8825     -23     
+ Misses      3063    3062      -1     
- Partials     237     239      +2     

Impacted Files	Coverage Δ
...rs/otlp/otlpmetric/internal/otlpmetrictest/data.go	0.0% <0.0%> (ø)
sdk/export/metric/metric.go	44.6% <0.0%> (+2.9%)	⬆️
sdk/metric/processor/basic/basic.go	98.4% <ø> (-0.1%)	⬇️
sdk/metric/processor/reducer/reducer.go	100.0% <ø> (ø)
sdk/metric/controller/basic/config.go	87.5% <50.0%> (ø)
sdk/metric/controller/basic/controller.go	90.9% <54.5%> (-3.1%)	⬇️
exporters/prometheus/prometheus.go	65.2% <83.3%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
exporters/otlp/otlpmetric/exporter.go	60.6% <100.0%> (ø)
...otlp/otlpmetric/internal/metrictransform/metric.go	79.0% <100.0%> (-1.0%)	⬇️
... and 5 more