fix missing metrics for notifications

[Issif]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup
/kind design
/kind documentation
/kind enhancement
/kind failing-test
/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area actionners
/area build
/area config
/area context
/area core
/area notifiers
/area ouputs
/area rule-engine

What this PR does / Why we need it:

The metric falcosecurity_falco_notifications_total wasn't increased

How to reproduce the issue:

Which issue(s) this PR fixes:

Special notes for your reviewer:

[IgorEulalio]

LGTM

[poiana]

LGTM label has been added.

Git tree hash: 26795cd1fa1fbf796ca00b2781aafb27422c738e

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: IgorEulalio, Issif

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