[processor/filter] Add functionality to allow filtering on SeverityNumber

[BinaryFissionGames]

Description:

• Add the ability to specify a minimum severity for filtering logs
  • Also added a flag that allows "undefined" severity logs to optionally be matched when min_severity is defined.

Link to tracking Issue: #12959

Testing:

• Added unit tests to test new functionality
• Manually tested with some mocked logs to check filtering

Documentation:

• Added documentation for new min_severity and match_undefined_severity fields
• Added example using new fields.

[djaglowski]

I'm not sure we need this option. The data model has an opinion on where "undefined" severity ranks vs other severities levels, so I think we should just support it according to that ordering.

[BinaryFissionGames]

I was really on the fence about this one, but IMO it's really useful from a user perspective, because "undefined" severity can come up in exceptional circumstances, e.g. I set up a filelog recevier, with a severity parsing operator. But maybe I mess up and typo one of the severities or something.

Now I'm dropping logs, because they are now undefined severity and I'm filtering everything below "info" level. But maybe those logs are actually really important, just the severity can't be parsed.

I think it's really nice to have this option when you need to be more fault-tolerant.

[djaglowski]

I can appreciate the use case, but I don't think we should have a separate setting. What do you think about reworking the config a bit?

log_severity:
  min: INFO
  match_undefined: true

[BinaryFissionGames]

Reworked the config as suggested; I used severity_number instead of log_severity to avoid stuttering and potential confusion with severity_texts.

[djaglowski]

LGTM

[djaglowski]

@boostchicken, do you want to take a look as code owner?