Allow step intervals at finer than whole second granularity for Wavefront

[shakuzen]

See #2173 where it was fixed that we were not using the configured step from WavefrontConfig to configure the Wavefront SDK's client. Until version 2.4 of the Wavefront SDK, it didn't allow setting a flushInterval in anything but whole seconds. It is not a common use case to need more than whole second granularity, but since we take an arbitrary Duration for step in the Config, we should align more closely with that to avoid confusing users. It will require the wavefront sdk used is version 2.4 or later, which we should call out in the release notes.