Make severity configurable in PrometheusRules

[aditya7302]

Fixes #1749

Made changes in pkg/sdk/logging/api/v1beta1/common_types.go to add a severity metrics in the Metrics struct and made changes in fluentbit, fluentd and syslog packages prometheusrules.go and buffervolumeprometheusrules.go .

[OverOrion]

Hey @aditya7302,

Thank you for your contribution, currently the CI fails, because the generated CRD files/docs have not been committed. Run the following to fix this:

make check-diff
git add -u
git commit

[pepov]

@aditya7302 can you please take a look at the original issue? Making the severity configurable alone is unfortunately not a good solution, see: #1749 (comment)

[pepov]

closing in favour of #1790 which is a more flexible solution