Breaks when using xdebug-2.9.1

[jason-linthwaite]

Additional environment details (Operating System, Cloud provider, etc):
Docker 18/19
php:7.2-fpm-alpine

Steps to reproduce the issue:

1. Include the following commands somwehere in docker file.
   1.1 RUN pecl install xdebug
   1.2 RUN curl -L -s https://github.com/DataDog/dd-trace-php/releases/download/0.36.0/datadog-php-tracer_0.36.0_noarch.apk --output datadog-php-tracer.apk
   1.3 RUN apk add datadog-php-tracer.apk --allow-untrusted
2. docker build .

Describe the results you received:
Image fails to build and error received:
`
[datadog]
extension=/opt/datadog-php/extensions/ddtrace-20170718.so
ddtrace.request_init_hook=/opt/datadog-php/dd-trace-sources/bridge/dd_wrap_autoloader.php

ddtrace.ini created

Created empty /opt/datadog-php/etc/ddtrace-custom.ini

Linking ddtrace.ini for supported SAPI's

Found SAPI config directory: /usr/local/etc/php/conf.d

Linking ddtrace.ini to /usr/local/etc/php/conf.d/98-ddtrace.ini

Linking ddtrace-custom.ini to /usr/local/etc/php/conf.d/99-ddtrace-custom.ini

Failed enabling ddtrace extension

`

Describe the results you expected:
Docker should build the image
DD-Agent should be enabled

Additional information you deem important (e.g. issue happens only occasionally):
For me, the reason for the error is the latest version of xdebug (2.9.1 released Jan 16th 2020). I changed / updated to use pecl install xdebug-2.9.0 and everything started working again.

Looks like some error or incompatibility with xdebug 2.9.1

Hope this helps.