Add new consent category for metrics data

[angelix]

Metrics data are sent only if session consent is given. If you don't want to track the session, it's impossible to get the metrics data.

This MR create a new category called "metrics" that used only to send metrics data. For backwards compatibility, metrics data are also sent if you give consent for "session".

Feel free to make any changes or just capture the idea behind this MR.

[angelix]

Maybe a change on the backend is required to accept metric data without begin_session.

[arifBurakDemiray]

Hello @angelix, We are going to close this PR due to it now being incompatible with the current state of the SDK. We have taken note of the idea of sending metrics separately and having a separate metric consent. Though to fully implement this, there are a couple of changes required on the server side to fully support this. We will come back to this idea at a future date. Thank you for your proposal. Have a great day.