Do you mean "keys" as in, your example, 'level', 'logger_name', 'process', and so on? If so, some variant of a search like index=fw
| fieldsummary
| fields field, count
| search count>0 NOT field IN ("date*", "eventtype", "index", "linecount", "punct", "source", "sourcetype", "splunk_server", "splunk_server_group", "tag*", "timeendpos", "timestartpos")
| stats count might do it. You'll have to remove the "stats count" at the end to double-check that all the Splunk built-in fields are removed in that `NOT field IN ....` part of the search.
... View more