management.metrics.export.signalfx properties are incomplete

[wilkinsona]

#33743 has revealed that the management.metrics.export.signalfx properties are incomplete. A property for pushCumulativeHistogram is missing.

[wilkinsona]

In 3.0.x, publishDeltaHistogram is also missing.

[wilkinsona]

Only one of delta or cumulative histograms may be published. If a SignalFxConfig implementation returns true from both publishCumulativeHistogram and publishDeltaHistogram, delta is used.

Rather than having two properties for this and relying on documentation for their mutual exclusivity, we could take advantage of this fact that we missed adding the properties and have an opportunity to consider both together by using a single property backed by an enum. In 2.7.x it would have two values (NONE, and CUMULATIVE). In 3.0.x we'd add DELTA to the values.