UI prevents adding 2 parameters to Percentile() function

[chasleslr]

Describe the bug
When attempting to add a Custom Expression with the Percentile() function, the docstring indicates that 2 parameters are expected, but the validation returns an error when adding 2 parameters with the following error:

Function Percentile expects 1 argument

Logs
These are the logs for when incorrectly attempting to run the Percentile() function with only 1 argument. The validation error appears to be entirely UI based and so I could not find any logs for it.

[679465e5-6615-4bcb-a3ed-b309bb259bdf] 2021-04-21T11:58:54-04:00 ERROR metabase.query-processor.middleware.catch-exceptions Error processing query: null
{:database_id 3,
 :started_at #t "2021-04-21T15:58:53.842786Z[GMT]",
 :error_type :invalid-query,
 :json_query
 {:database 3,
  :query
  {:source-table 835, :aggregation [["aggregation-options" ["percentile" ["field" 10461 nil]] {:display-name "95%"}]]},
  :type "query",
  :parameters [],
  :middleware {:js-int-to-string? true, :add-default-userland-constraints? true}},
 :native nil,
 :status :failed,
 :class clojure.lang.ExceptionInfo,
 :stacktrace
 ["--> util.schema$schema_core_validator$fn__16373.invoke(schema.clj:29)"
  "query_processor.middleware.validate$validate_query$fn__49352.invoke(validate.clj:9)"
  "query_processor.middleware.normalize_query$normalize$fn__47233.invoke(normalize_query.clj:22)"
  "query_processor.middleware.add_rows_truncated$add_rows_truncated$fn__44627.invoke(add_rows_truncated.clj:35)"
  "query_processor.middleware.results_metadata$record_and_return_metadata_BANG_$fn__49283.invoke(results_metadata.clj:147)"
  "query_processor.middleware.constraints$add_default_userland_constraints$fn__46478.invoke(constraints.clj:42)"
  "query_processor.middleware.process_userland_query$process_userland_query$fn__48395.invoke(process_userland_query.clj:135)"
  "query_processor.middleware.catch_exceptions$catch_exceptions$fn__46418.invoke(catch_exceptions.clj:173)"
  "query_processor.reducible$async_qp$qp_STAR___37956$thunk__37957.invoke(reducible.clj:103)"
  "query_processor.reducible$async_qp$qp_STAR___37956.invoke(reducible.clj:109)"
  "query_processor.reducible$sync_qp$qp_STAR___37965$fn__37968.invoke(reducible.clj:135)"
  "query_processor.reducible$sync_qp$qp_STAR___37965.invoke(reducible.clj:134)"
  "query_processor$process_userland_query.invokeStatic(query_processor.clj:239)"
  "query_processor$process_userland_query.doInvoke(query_processor.clj:235)"
  "query_processor$fn__49442$process_query_and_save_execution_BANG___49451$fn__49454.invoke(query_processor.clj:251)"
  "query_processor$fn__49442$process_query_and_save_execution_BANG___49451.invoke(query_processor.clj:243)"
  "query_processor$fn__49486$process_query_and_save_with_max_results_constraints_BANG___49495$fn__49498.invoke(query_processor.clj:263)"
  "query_processor$fn__49486$process_query_and_save_with_max_results_constraints_BANG___49495.invoke(query_processor.clj:256)"
  "api.dataset$run_query_async$fn__55699.invoke(dataset.clj:56)"
  "query_processor.streaming$streaming_response_STAR_$fn__55678$fn__55679.invoke(streaming.clj:72)"
  "query_processor.streaming$streaming_response_STAR_$fn__55678.invoke(streaming.clj:71)"
  "async.streaming_response$do_f_STAR_.invokeStatic(streaming_response.clj:65)"
  "async.streaming_response$do_f_STAR_.invoke(streaming_response.clj:63)"
  "async.streaming_response$do_f_async$fn__16071.invoke(streaming_response.clj:84)"],
 :context :ad-hoc,
 :error
 "Value does not match schema: {:query {:aggregation [[nil (named (named [nil nil (named (named (not (some-matching-condition? nil)) \"Must be a valid instance of one of these clauses: :expression, :field\") \"percentile\")] \"Must be a valid instance of one of these clauses: :count, :avg, :cum-count, :cum-sum, :distinct, :stddev, :sum, :min, :max, :metric, :share, :count-where, :sum-where, :case, :median, :percentile, :var\") \"aggregation\") nil]]}}",
 :row_count 0,
 :running_time 0,
 :preprocessed nil,
 :ex-data
 {:type :schema.core/error,
  :value
  {:database 3,
   :query
   {:source-table 835,
    :aggregation [[:aggregation-options [:percentile [:field 10461 nil] nil] {:display-name "95%"}]]},
   :type :query,
   :middleware {:js-int-to-string? true, :add-default-userland-constraints? true},
   :info
   {:executed-by 1,
    :context :ad-hoc,
    :nested? false,
    :query-hash
    [-114, 98, -95, 94, -45, -59, -110, -112, -10, 81, -24, -41, 92, -104, 116, 114, -52, 16, -54, 89, -121, 36, -121,
     -65, -29, -4, 27, 10, 57, -62, 46, 60]},
   :constraints {:max-results 10000, :max-results-bare-rows 2000}},
  :error
  {:query
   {:aggregation
    [[nil
      (named (named [nil nil (named (named (not (some-matching-condition? nil)) "Must be a valid instance of one of these clauses: :expression, :field") "percentile")] "Must be a valid instance of one of these clauses: :count, :avg, :cum-count, :cum-sum, :distinct, :stddev, :sum, :min, :max, :metric, :share, :count-where, :sum-where, :case, :median, :percentile, :var") "aggregation")
      nil]]}}},
 :data {:rows [], :cols []}}

To Reproduce
Steps to reproduce the behavior:

1. Create a new Question
2. Create a new Custom Expression using the Percentile() function
3. Attempt to add a column, and a percentile-value (as shown in the docstring)
4. See error: Function Percentile expects 1 argument

Expected behavior
I should be able to add 2 parameters to the Percentile() function.

Screenshots

Adding 1 parameter correctly validates in the UI, however, it returns an error when attempting to run:

Information about your Metabase Installation:

You can get this information by going to Admin -> Troubleshooting.

• Your browser and the version: Chrome/89.0.4389.128
• Your operating system: Mac OS X 10.15.7
• Your databases: Snowflake
• Metabase version: v0.39.0
• Metabase hosting environment: Elastic Beanstalk
• Metabase internal database: Postgres

Severity
This is completely preventing the use of the Percentile function.

[flamber]

Works on 0.38.4, but not on 0.39.0.1 - must have happened during an update to the Custom Expression formatter.

[ariya]

This is due the args mistake in frontend/src/metabase/lib/expressions/config.js (this data was ignored before 39, hence why the front-end did not complain before):

  percentile: {
    displayName: `Percentile`,
    type: "aggregation",
    args: ["number"],
    requiresFeature: "percentile-aggregations",
  },

I have a fix coming up soon.