Access field in Map from within metric


Does the metrics engine support accessing fields from a Map within a simple metric expression? For example, if I have a type as:

entity type MyType {
  self: [MyType](id, id)

  params: map<string, any> 


and within params I always have a value for paramA, is there a way to define a metric along the lines of:

    "id": "ParamAMetric_MyType",
    "name": "ParamAMetric",
    "srcType": "MyType",
    "path": "self",
    "expression": "identity(params['paramA'])"

I have tried a few things along the lines of the above without any luck