Add a way to drop histogram bucket metrics with the SignalFx exporter

[samiura]

Component(s)

exporter/signalfxexporter

Describe the issue you're reporting

To support histogram metrics currently, we would want to add a way for the signalfx exporter to drop bucket metrics instead of sending them as separate MTS as currently. This option should be disabled by default.

[github-actions]

Pinging code owners for exporter/signalfx: @dmitryax @crobert-1. See Adding Labels via Comments if you do not have permissions to add labels yourself.