Add support of Graylog GELF Alerter

[malinkinsa]

Description

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

[jertel]

Thanks for the submission. I recommend looking at the tests/alerters/ directory, as you'll probably find a template there to use for adding unit test coverage.

[malinkinsa]

Yep, till the end of the week will do it

[malinkinsa]

@jertel
Done

[jertel]

Thanks for adding the unit tests. Will you please also update the schema.yaml file, with the expected parameter data types? Search for "discord" or "alerta" for examples of what is needed.

[malinkinsa]

update the schema.yaml file

Ready.

@nsano-rururu many thanks for your edits, with them it is better.

[nsano-rururu]

About settings Won't you add validation settings that are referenced when rules are executed? If the data type and input value variations are fixed, add settings.
https://github.com/jertel/elastalert2/blob/master/elastalert/schema.yaml

example

gelf_ignore_ssl_errors: {type: boolean}
gelf_type: {type: string, enum: ['http', 'tcp']}
gelf_version: {type: string}

[nsano-rururu]

Is gelf_log_level a number? Have you decided on a range? If the minimum and maximum are fixed, we recommend adding the following settings to schema.yaml

[malinkinsa]

About gelf_version - i didn't want to add a specified version list to schemas. The latest version is 1.1 and it is default now and this version was released in 2013.

About gelf_log_level - good point. I'm adding a range now.