metric exporter: nothing to do for an empty list of metrics

[tsloughter]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.19%. Comparing base (bda6844) to head (b314453).
Report is 76 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #782   +/-   ##
=======================================
  Coverage   73.19%   73.19%           
=======================================
  Files          64       64           
  Lines        1940     1940           
=======================================
  Hits         1420     1420           
  Misses        520      520           

Flag	Coverage Δ
api	69.90% <ø> (ø)
elixir	17.32% <ø> (ø)
erlang	74.51% <ø> (ø)
exporter	73.06% <ø> (ø)
sdk	77.30% <ø> (ø)
zipkin	54.16% <ø> (ø)

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