Feature: Early metrics POST

[FredrikOseberg]

Describe the feature request

In the Javascript SDK, we have added a configurable parameter which indicates how long after startup the SDK should send the first metrics. The default in the javascript SDK is set to two seconds. Since none of our client side SDKs register, the metrics is the first touchpoint for when Unleash can "see" your application and toggle usage.

We'd like to align the API surface across our SDKs and add support in the Swift SDK for sending early metrics on a configurable interval defaulted to two seconds, similar to the Javascript SDK.

Reference implementation: https://github.com/Unleash/unleash-proxy-client-js/pull/203/files
Javscript SDK properties overview: https://github.com/Unleash/unleash-proxy-client-js/pull/203/files

[stale]

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