Gets the measurement key strings of measurements within the given value.

val The value which has the measurement key.
allow_decompose Defaults to True. When true, composite operations that don't directly specify their measurement keys will be decomposed in order to find measurement keys within the decomposed operations. If not set, composite operations will appear to have no measurement keys. Used by internal methods to stop redundant decompositions from being performed.

The measurement keys of the value. If the value has no measurement, the result is the empty set.