Add publishAverage config option to Datadog registry

[jaxxnitt]

This PR implements the enhancement discussed in #6844.

Summary

Adds a new publishAverage() boolean config option to DatadogConfig (default: true).

Updates DatadogMeterRegistry to conditionally publish .avg metrics for:

Timer

FunctionTimer

DistributionSummary

Ensures metadata for average metrics is also controlled by this flag.

Keeps existing 1.x behavior by default, while allowing users to disable average publishing if desired.

This aligns with the intended direction for 1.x and prepares the way for flipping the default in 2.x as discussed in #6843.

Feel free to suggest changes.