Imroved Heroku Log Severity parsing

Added

By default, the Heroku LogDrain emits all messages with an "Info" severity, and while we support overriding the severity by using custom attributes in either Logfmt or JSON format, it's not always possible to do this.

When creating (or updating) a Heroku log source, there's now an option to enable "Parse severity from a log message."

This option will attempt to parse the severity from any log message that does not contain a severity attribute, and allows you to get add-on and 3rd party logs into AppSignal with the correct severity.

You can find more information about what severity strings are parsed and how on our Heroku LogDrain documentation.