Exporter/Metrics/OcAgent: Optimization for already-sent metrics.

[songy23]

As a follow-up of #1629.

For already-sent metrics, set name instead of metric descriptor and don't need to set bucket options.

Updates #1567.

[codecov-io]

Codecov Report

Merging #1637 into master will increase coverage by 0.03%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master    #1637      +/-   ##
============================================
+ Coverage     83.03%   83.07%   +0.03%     
- Complexity     1851     1853       +2     
============================================
  Files           265      265              
  Lines          8551     8553       +2     
  Branches        823      824       +1     
============================================
+ Hits           7100     7105       +5     
+ Misses         1185     1182       -3     
  Partials        266      266

Impacted Files	Coverage Δ	Complexity Δ
...us/exporter/metrics/ocagent/MetricsProtoUtils.java	85.82% <85.71%> (+0.22%)	27 <0> (+2)	⬆️
...census/implcore/trace/export/SpanExporterImpl.java	95.89% <0%> (+1.36%)	9% <0%> (ø)	⬇️
...e/ocagent/OcAgentTraceServiceConfigRpcHandler.java	79.31% <0%> (+3.44%)	12% <0%> (ø)	⬇️

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[bogdandrutu]

Let's make sure we are ok to have this optimization. In the previous PR I was suggesting to just optimize the conversion of Descriptor and BucketsOptions to proto.

[songy23]

I see, though I think this is why we add the metric.name field in the proto:

// Defines a Metric which has one or more timeseries.
message Metric {
  // The descriptor of the Metric. This is an optimization for network wire
  // size, from data-model perspective a Metric contains always a
  // MetricDescriptor.
  oneof descriptor {
    // In case of a streaming RPC can be sent only the first time a metric is
    // reported to save network traffic.
    MetricDescriptor metric_descriptor = 1;

    // In case of a streaming RPC this can be sent for metrics that already
    // sent the MetricDescriptor once.
    string name = 2;
  }
...
}

[songy23]

We haven't made a decision on census-instrumentation/opencensus-proto#152 yet. Closing this PR for now.