exporter/signalfx: Update exclude_metrics option

[asuresh4]

Description: Update the behavior of exclude_metrics option. The PR updates the behavior so that the default exclude rules can be overridden by setting exclude_metrics to []. Currently, the exporter appends a list of default exclude rules. This update will mean that the exporter only appends default rules for if exclude_metrics is not specified or is a non-empty slice.

Link to tracking Issue:

Testing: Added tests.

Documentation: Updated README

[codecov]

Codecov Report

Merging #2737 (5a81216) into main (f8c550f) will increase coverage by 1.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2737      +/-   ##
==========================================
+ Coverage   90.53%   91.59%   +1.05%     
==========================================
  Files         444      445       +1     
  Lines       21984    22058      +74     
==========================================
+ Hits        19904    20203     +299     
+ Misses       1618     1385     -233     
- Partials      462      470       +8     

Flag	Coverage Δ
integration	69.18% <ø> (?)
unit	90.52% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/signalfxexporter/factory.go	82.45% <100.00%> (-0.31%)	⬇️
receiver/k8sclusterreceiver/watcher.go	95.29% <0.00%> (-2.36%)	⬇️
processor/groupbytraceprocessor/event.go	95.96% <0.00%> (-0.81%)	⬇️
internal/common/testing/container/container.go	73.68% <0.00%> (ø)
receiver/jmxreceiver/receiver.go	94.80% <0.00%> (+16.88%)	⬆️
receiver/dockerstatsreceiver/docker.go	92.30% <0.00%> (+39.05%)	⬆️
receiver/dockerstatsreceiver/receiver.go	96.72% <0.00%> (+47.54%)	⬆️
receiver/jmxreceiver/subprocess/subprocess.go	96.55% <0.00%> (+74.48%)	⬆️
receiver/redisreceiver/receiver.go	87.50% <0.00%> (+87.50%)	⬆️
receiver/redisreceiver/client.go	100.00% <0.00%> (+100.00%)	⬆️
... and 2 more

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