Avoid hardcoding Segment write key

[kim-tsao]

Which area/kind this issue is related to?

/area registry
/area telemetry

Issue Description

Users setting up their own index server could turn on telemetry and that would result in data being sent to our servers since the write key is hardcoded. We should avoid this scenario and ensure the write key is set in some environment variable, either specified by the user or by us.

[kim-tsao]

As discussed, we will be removing the enabled field since it's not necessary anymore. If a user specifies a telemetry key then we can assume they want telemetry to be enabled.