Not supporting for PHP7.4

[kishoresai438]

Hi Team,

I have upgraded my application to php7.4. Opencensus stable version not supporting php7.4. So, I have updated opencensus to dev-master version. But for this, Stackdriver exporter not supporting Opencensus dev-master version. Can you please provide support for this. Thanks.

[hakimdjabri]

Is this package unmaintained? Should we look for another method to get PHP tracing on Google cloud?