Create an Inmemory Exporter for test

[MadVikingGod]

This change creates an in-memory exporter for use in tests.

Closes: #2722

This affects a number of instruments in the contrib repo. An example of what this can re-enable is: MadVikingGod/opentelemetry-go-contrib@1e9dd43

[codecov]

Codecov Report

Merging #2776 (fea1be2) into main (c05c3e2) will increase coverage by 0.0%.
The diff coverage is 70.5%.

@@          Coverage Diff           @@
##            main   #2776    +/-   ##
======================================
  Coverage   75.7%   75.7%            
======================================
  Files        178     180     +2     
  Lines      11707   11818   +111     
======================================
+ Hits        8864    8951    +87     
- Misses      2618    2634    +16     
- Partials     225     233     +8     

Impacted Files	Coverage Δ
sdk/metric/metrictest/exporter.go	68.0% <68.0%> (ø)
sdk/metric/metrictest/config.go	82.3% <82.3%> (ø)
sdk/metric/aggregator/aggregatortest/test.go	81.0% <100.0%> (ø)
sdk/trace/batch_span_processor.go	81.1% <0.0%> (+0.9%)	⬆️
sdk/metric/sdkapi/wrap.go	92.8% <0.0%> (+8.3%)	⬆️

[dashpole]

This looks good, and is definitely a useful starting point.

Having used prometheus a bunch for metrics, i've come to really like the GatherAndCompare (to a file) approach for testing. Its nice to have metrics represented in files if I need to do a large number of assertions about my metrics. It would be neat to be able to make assertions against a json OTLP file or something. Or, I guess, I can just use the prom exporter with GatherAndCompare to meet my use-case :)...