Metrics support

[Tymek]

Describe the feature request

Support for metrics and app registration in server-side part of this SDK. In serverless/edge environments it's different then what we had before.

Background

Users who enabled metrics when creating a feature toggle expect it to show up.

Solution suggestions

We have 2 options:

A. Do it as-is, sending a request on every evaluation, but it will not be performant (not scalable).
B. Research alternative ways of batching metrics. Not a trivial task.

[Tymek]

New beta version has experimental support for metrics.

https://github.com/Unleash/unleash-client-nextjs?tab=readme-ov-file#server-side-flags-and-metrics-experimental-since-150-beta3

[Tymek]

Released with v1.5. Please provide feedback if you use this feature, especially if you encounter any issues.