Deprecate library

[yurishkuro]

@jcchavezs most other OpenTracing APIs are tagged as deprecated. Please consider doing the same for this one.

[lindsaygrace]

With the php-open-telemetry library being in the state it is, there isn't much of an alternative at the moment.

[brettmc]

Hello! OpenTelemetry for PHP is now deep into beta, and getting close to GA.
We've just released an initial version of an Opentracing-compatible shim on packagist and would appreciate some testing (which should ease the transition from opentracing to opentelemetry).

[brettmc]

Hi @lindsaygrace
It took us a long time, but OpenTelemetry for PHP has today tagged 1.0.0
I hope you will consider marking this repo as deprecated and suggesting users towards us as a replacement :)

Thanks for the inspiration - when I looked at this code, it was obvious how heavily early OpenTelemetry for PHP drew from this library!