otelgrpc: Set "rpc.server.duration" metric description

[puckpuck]

Fixes #4301

This sets the description for the rpc.server.duration metric. This description is set as defined by the semantic convention for this metric.

When used with other services written in other languages that emit the same metric, all services must emit the same metric metadata (name, description) when used in conjunction with a Prometheus backend. The OpenTelemetry Collector, Prometheus exporter, will drop metrics that match the name but not the description against already received metrics. We have faced this problem as part of the OpenTelemetry demo project.

[codecov]

Codecov Report

Merging #4302 (1e3080b) into main (da0d2ea) will increase coverage by 0.0%.
The diff coverage is 75.0%.

@@          Coverage Diff          @@
##            main   #4302   +/-   ##
=====================================
  Coverage   81.9%   81.9%           
=====================================
  Files        141     141           
  Lines       9708    9711    +3     
=====================================
+ Hits        7960    7963    +3     
  Misses      1616    1616           
  Partials     132     132           

Files Changed	Coverage
...entation/google.golang.org/grpc/otelgrpc/config.go	75.0%