Updated GelfFormatter/GelfHandler

[bzikarsky]

Change implementation to use bzikarsky/gelf-php as discussed in Issue #258

[Seldaek]

Thanks! As said though I can't really merge this before 2.0, so it might be a little while.

[bzikarsky]

I totally understand. :-)

[stof]

wrong package name here

[bzikarsky]

True. Will fix that. Thanks!

[bzikarsky]

Well, this took more commits than it should have. Do you want me to squash?

[Seldaek]

If you are done with everything sure feel free to squash it

[bzikarsky]

squashed & pushed. :-)

[bzikarsky]

Since this came up several times:

Monolog 2.0 seems to be far away, but people like to switch to graylog2/gelf-php. mlehner's fork is not actively developed anymore in favor of the official lib.

What do you think of those 2 backwards compatible changes?

1. Drop the constructor typehint for IMessagePublisher in https://github.com/Seldaek/monolog/blob/master/src/Monolog/Handler/GelfHandler.php#L36 and do a runtime check for either Gelf\IMessagePublisher or Gelf\PublisherInterface
2. Library-specific implementation of GelfMessageFormatter::format
   We can identify the Gelf-library in use by the existence of Gelf\IMessagePublisher vs Gelf\PublisherInterface
   https://github.com/mlehner/gelf-php/blob/master/src/Gelf/IMessagePublisher.php
   https://github.com/bzikarsky/gelf-php/blob/master/src/Gelf/PublisherInterface.php

/cc @h4cc

[bzikarsky]

Addendum: 2. is unneccessary. The GelfMessageFormatter is compatible with both graylog2/gelf.php and mlehner/gelf-php.

So it's just a change to the constructor of the GelfHandler. To verify the functionality I would add an additional GelfHandlerLegacyTest class, which gets executed when mlehner's fork is installed.

[bzikarsky]

I'll close this PR, and open a new one with an updated description.