public final FieldValue setFieldValue(String field, Long value) {
        return setFieldValue(field, new LongFieldValue(value));
    }
Convenience method to set the value of a named field. This is the same as calling {@link #getField(String)}, and
using the returned value to call {@link #setFieldValue(Field, FieldValue)}. If the named field does not exist,
this method returns null.

@param fieldName The name of the field whose value to set.
@param value     The value to set.
@return The previous value of the field, or null.
document/src/main/java/com/yahoo/document/datatypes/StructuredFieldValue.java