change UploadMetrics signature from slice to single Resource

[hanyuancheung]

Fix: #2228

• UploadMetrics() only ever receiving a single Resource, instead of taking []*metricpb.ResourceMetrics slice.
• Change the code accordingly. Change its signature from slice to single one.

[codecov]

Codecov Report

Merging #2491 (07344a0) into main (776bfdc) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2491     +/-   ##
=======================================
- Coverage   76.3%   76.3%   -0.1%     
=======================================
  Files        173     173             
  Lines      12022   12022             
=======================================
- Hits        9182    9179      -3     
- Misses      2597    2598      +1     
- Partials     243     245      +2     

Impacted Files	Coverage Δ
exporters/otlp/otlpmetric/exporter.go	60.6% <100.0%> (ø)
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	97.7% <100.0%> (ø)
exporters/otlp/otlpmetric/otlpmetrichttp/client.go	78.7% <100.0%> (ø)
sdk/metric/refcount_mapped.go	80.0% <0.0%> (-20.0%)	⬇️
sdk/metric/sdk.go	80.0% <0.0%> (-1.5%)	⬇️
sdk/trace/batch_span_processor.go	85.4% <0.0%> (+2.0%)	⬆️