[cmd/otelinmemexporter] Add histogram support

[ericywl]

🧑‍💻 What is being changed

• Add histogram support for otelinmemexporter. Aggregations for histogram includes percentile and sum.
• Refactor / rename some existing functions, fields etc.

✅ How to validate the change

1. Configure ocb-config.yaml to use local path for otelinmemexporter and rebuild the collector. For example:

exporters:
  - gomod: go.opentelemetry.io/collector/exporter/otlpexporter v0.120.0
  - gomod: github.com/elastic/apm-perf/cmd/otelinmemexporter v0.0.0-20241111024659-ec5113ead30e
    path: /Users/ericyap/Repos/apm/apm-perf/cmd/otelinmemexporter

processors:
  - gomod: go.opentelemetry.io/collector/processor/batchprocessor v0.120.0
  - gomod: github.com/open-telemetry/opentelemetry-collector-contrib/processor/cumulativetodeltaprocessor v0.120.0

receivers:
  - gomod: go.opentelemetry.io/collector/receiver/otlpreceiver v0.120.0

2. Configure aggregation for some histogram.

3. Run benchmark (with some changes to otelinmemexporter to log / print out histogram aggregations). Example output:

{"latency":{"":{"resourceMetrics":[{"resource":{},"scopeMetrics":[{"scope":{},"metrics":[{"histogram":{"dataPoints":[{"attributes":[{"key":"partition","value":{"stringValue":"0"}},{"key":"project_id","value":{"stringValue":"d8b2b3711fa948518f497173e850fe19"}}],"startTimeUnixNano":"1739948581207620451","timeUnixNano":"1739948591207399095","count":"43","sum":265.11085513400394,"bucketCounts":["0","27","5","11","0","0","0","0","0","0","0","0","0","0","0","0"],"explicitBounds":[0,5,10,25,50,75,100,250,500,750,1000,2500,5000,7500,10000]}]}}]}]}]}}}
map[string]float64{"":22.068181818181817}

{"latency":{"":{"resourceMetrics":[{"resource":{},"scopeMetrics":[{"scope":{},"metrics":[{"histogram":{"dataPoints":[{"attributes":[{"key":"partition","value":{"stringValue":"0"}},{"key":"project_id","value":{"stringValue":"d8b2b3711fa948518f497173e850fe19"}}],"startTimeUnixNano":"1739948581207620451","timeUnixNano":"1739948601207784807","count":"93","sum":438.06849422600135,"bucketCounts":["0","77","5","11","0","0","0","0","0","0","0","0","0","0","0","0"],"explicitBounds":[0,5,10,25,50,75,100,250,500,750,1000,2500,5000,7500,10000]}]}}]}]}]}}}
map[string]float64{"":18.659090909090903}

{"latency":{"":{"resourceMetrics":[{"resource":{},"scopeMetrics":[{"scope":{},"metrics":[{"histogram":{"dataPoints":[{"attributes":[{"key":"partition","value":{"stringValue":"0"}},{"key":"project_id","value":{"stringValue":"d8b2b3711fa948518f497173e850fe19"}}],"startTimeUnixNano":"1739948581207620451","timeUnixNano":"1739948611207250531","count":"137","sum":632.7414063080014,"bucketCounts":["0","119","7","11","0","0","0","0","0","0","0","0","0","0","0","0"],"explicitBounds":[0,5,10,25,50,75,100,250,500,750,1000,2500,5000,7500,10000]}]}}]}]}]}}}
map[string]float64{"":15.659090909090901}

4. It should also appear in apmbench output. For example:

BenchmarkAgentAll-4	     618	 828864153 ns/op	         4.750 latency ...
BenchmarkAgentAll-4	     427	 746870351 ns/op	     	 4.765 latency ...

[ericywl]

Will merge main over after #230 is merged.

[1pkg]

The code looks good and very clean, left just a few nits.
Since it's a larger change and it's easy to lose something, I would recommend to get at least a couple approvals before merging.