[exporter/signalfx] attempts to divide by zero during translation are logged as warnings

[pmcollins]

Describe the bug
The SignalFx exporter can produce noisy log entries about translation rules finding a zero in the denominator in what would be a division operation. The operation is skipped, so no spurious values are actually created, but the warning that gets logged has been reported to be a hindrance that is not actionable to users.

Steps to reproduce
Unable to reproduce organically, but if you send cpu utilization counter metrics whose aggregate value doesn't change into the collector with default SFx translation rules, the translator will calculate a delta of zero of total cpu time available, and cause an attempt to divide by zero, causing the warning to get logged.

What did you expect to see?
The event to be logged at a lower level.

What did you see instead?
The event was logged at a warning level.

What version did you use?
0.56.0

What config did you use?
A vanilla/default config for the SFx exporter.

Environment
Debian 11

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