fix stats for Mattermost and add tests

[cpanato]

What type of PR is this?
/kind cleanup

Any specific area of the project related to this PR?
/area outputs

What this PR does / why we need it:

• fix the stats for Mattermost which was using for another receiver.
• add some tests
• format a bit

ps. I can add tests for the other if no one is against

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

[poiana]

@cpanato: The label(s) area/outputs cannot be applied, because the repository doesn't have them

In response to this:

What type of PR is this?
/kind cleanup

Any specific area of the project related to this PR?
/area outputs

What this PR does / why we need it:

• fix the stats for Mattermost which was using for another receiver.
• add some tests
• format a bit

ps. I can add tests for the other if no one is against

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[poiana]

Welcome @cpanato! It looks like this is your first PR to falcosecurity/falcosidekick 🎉

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Issif

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• OWNERS [Issif]

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[poiana]

LGTM label has been added.

Git tree hash: 1c7e7cdb716d1c1b897b140e9a91b068d469178f