Phoenix 1.5 removes the instrumenters API

[garthk]

The instrumenters API is gone:

• phoenixframework/phoenix@19b0b01
• phoenixframework/phoenix@ec78247

We'll need to use :telemetry, instead:

• phoenixframework/phoenix@4d61eac

[hauleth]

This was meant as a temporary solution for monitoring anyway. After 1.5 we can just deprecate this and redirect users to opencensus_telemetry.