I'm currently using an older (6months+) version of esri-leaflet and previous I modified the onEachFeature function to be able to find the related L.esri.featureLayer for the layer.
With this I could then access the field metadata through featureLayer.fields
The reason was for simple type detection and formatting for display in popup - i.e. detect the esriFieldTypeOID field and ignore it, or detect a date field and format the output accordingly etc.
I don't see an equivalent of the 'fields' array in the current FeatureLayer class - is there somewhere else I can get this metadata for a layer?