Disable metrics when active=false

[felixbarny]

Currently, even if the agent is deactivated, it still sends metrics. We should change that.

See also https://discuss.elastic.co/t/unable-to-disable-apm-via-environment-variable/180558

[eyalkoren]

I think this is a rethink on what active means and how it is enforced. For example- if set on startup, should we do health check? Should we even establish a connection?
Definitely requires updating the documentation for this property in any case.