AWS::Logs::MetricFilter MetricValues permits invalid strings

[trav-c]

cfn-lint 0.30.1

AWS::Logs::MetricFilter.Properties.MetricTransformations[*].MetricValue allows a bare string not starting with '$' which it appears is never actually valid, ie
"MetricValue: length" vs "MetricValue: $length"

Assuming I'm reading the documentation correctly MetricValue must always either be a number OR start with a '$' character.

The following fragment lints, but is rejected by CloudFormation at runtime without a $ at the start of the named MetricValue field

    QueueLengthMetricFilter:
        Type: AWS::Logs::MetricFilter
        Properties:
            LogGroupName: !Ref LogGroup
            FilterPattern: '[date, time, tag="rh-sched*", x01=throttling, x02="jobs.", ..., x10=Len, x11=of, x12=job, x13="queue*",  length]'
            MetricTransformations:
              - MetricValue: length
                MetricNamespace: !Sub '${EnvironmentName}'
                MetricName: 'JobsQueued'

Note: I believe that this is also missed by the AWS ValidateTemplate API

[PatMyron]

"For example, if you're counting the occurrences of a particular term like Error, specify 1 for the metric value. If you're counting the number of bytes transferred, reference the value that is in the log event by using $ followed by the name of the field that you specified in the filter pattern, such as $size."

---

As described here, this would need an AllowedPatternRegex to be enforced:

https://github.com/aws-cloudformation/cfn-python-lint/blob/a46773c752247c51effef415bd3462eaec10ab0b/src/cfnlint/data/ExtendedSpecs/all/03_value_types.json#L33-L36

[ghost]

This is still an issue, would love to see it fixed.