add exemplar reservoir support

[tsloughter]

Opening as a draft because it only has SimpleFixed reservoir and it has no configuration for whether to use exemplars and how to filter.

Only the drop and simple fixed exemplar reservoir are supported by this PR, so patchstograms do not yet have exemplar support.

This patch also does not have a toggle for if exemplars are to be taken or not.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b746f08) 73.02% compared to head (52ac8d9) 73.11%. Report is 3 commits behind head on main.

Files	Patch %	Lines
apps/opentelemetry/src/otel_configuration.erl	66.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #692      +/-   ##
==========================================
+ Coverage   73.02%   73.11%   +0.09%     
==========================================
  Files          61       61              
  Lines        1924     1927       +3     
==========================================
+ Hits         1405     1409       +4     
+ Misses        519      518       -1     

Flag	Coverage Δ
api	69.92% <ø> (+0.28%)	:arrow_up:
elixir	17.47% <ø> (ø)
erlang	74.44% <66.66%> (+0.09%)	:arrow_up:
exporter	67.47% <ø> (ø)
sdk	78.65% <66.66%> (-0.04%)	:arrow_down:
zipkin	54.16% <ø> (ø)

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