Can't use Len() function with almost any slice

[obs-gh-enricogiorio]

I'm trying to do processing of k8s events and I need to calculate the length of some slices (fields of some k8s objects).
I just don't understand why I can't calculate the length of any slice. For instance, why do I need to implement a custom processor if I have to calculate the length of this slice? My understanding is that OTTL works on the pcommon.Logs and that Slice has Len() implemented here https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/processor/deltatocumulativeprocessor/internal/putil/pslice/pslice.go#L6.
But when I use it I get:

 warn    ottl/parser.go:321      failed to execute statement     {"kind": "processor", "name": "transform/object", "pipeline": "logs/objects", "error": "target arg must be of type string, []any, map[string]any, pcommon.Map, pcommon.Slice, pcommon.Value (of type String, Map, Slice) or a supported slice type from the plog, pmetric or ptrace packages", "statement": "set(attributes[\"observe_transform\"][\"facets\"][\"imagePullSecrets\"], Len(body[\"imagePullSecrets\"]))", "trace_id": "34efd4b49042d6c14d26f44f03e91ebe", "span_id": "53da2d6a7c01f204"}