[connector/signaltometrics] Implement optional attribute filtering

[lahsivjar]

Current attribute filtering performed by signal to metrics connector is:

1. If default_value is specified then ALWAYS produce the metric for the defined configuration. If the attribute is not present in the incoming data then the default value is used else the attribute value from the incoming data is used.
2. If default_value is NOT specified then only produce the metric for the defined configuration if the incoming data as all the attributes, without default_value, available in the incoming data.

There are cases when we want a third option i.e. an optional attribute. If the attribute is present in the incoming data then include it, else, don't include it. This basically means that there would be another possible classification bucket with that attribute missing.