Are PDAL or PCL capable of producing LiDAR grid metrics?

PDAL doesn't provide anything like FUSION's "GridMetrics" at this time. We've been interested in useful statistics or metrics that PDAL could compute for algorithm builders, but we haven't gotten around to implementing anything yet. It would be straightforward to implement a custom PDAL stage to compute these. It will be more productive to ask on the mailing list for more information.

You could consider using SPDLib, which has a command called spdmetrics. It can be used to simultaneously calculate multiple vegetation metrics and output them in either raster or vector formats (individual polygons).

There is a tutorial available here that explains how the metrics can be calculated and this XML file provides a full list of the metric primitives and operators.