ESQL: Replace function count tests (elastic#134951)

Replaces the tests for a count of functions with a spot check of some
specific functions. `delay` should never be released. `idelta` isn't
released but will be. `mv_sum` has been released. It's nice to have the
ones we want to make sure aren't available in production in there, but
otherwise it's a fairly random list.

Doing this because maintaining the count was constantly a problem -
folks would stumble over it all the time and it wasn't really buying us
anything.

Author: nik9000

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/package-info.java

@@ -223,9 +223,11 @@
  *         When it comes to the correct syntax, e.g.: annotations, groupings, etc., feel free to look at existing snapshot functions and
  *         Git history.
  *
- *         Depending on whether the new function requires a snapshot build, you may need to increment the expected ES|QL functions count in
- *         one or both places in {@code x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/60_usage.yml}. You can also run
- *         these tests locally, just make sure to pass the right flags so you can toggle between snapshot and non-snapshot tests.
+ *         If your function should be available in snapshot <strong>only</strong>, add it to the list in
+ *         {@code x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/60_usage.yml} around
+ *         {@code not_exists: esql.functions.delay}. The release build will fail if this function is
+ *         available. Check out the instructions for running a release build in {@code testing.asciidoc}
+ *         if you want to test this, but it's generally enough to let CI do it.
  *     </li>
  *     <li>
  *         Open the PR. The subject and description of the PR are important because those'll turn

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/60_usage.yml

@@ -133,8 +133,13 @@ setup:
   - gt: { esql.functions.to_long: $functions_to_long }
   - match: { esql.functions.coalesce: $functions_coalesce }
   - gt: { esql.functions.categorize: $functions_categorize }
-  # Testing for the entire function set isn't feasible, so we just check that we return the correct count as an approximation.
-  # - length: {esql.functions: 179} # check the "sister" test below for a likely update to the same esql.functions length check
+
+  # There's one of these per function but that's a ton of things to check. So we just spot check that a few exist.
+  - exists: esql.functions.delay
+  - exists: esql.functions.idelta
+  - exists: esql.functions.mv_sum
+  - exists: esql.functions.to_dateperiod
+
 ---
 "Basic ESQL usage output (telemetry) non-snapshot version":
   - requires:
@@ -231,7 +236,12 @@ setup:
   - gt: { esql.functions.to_long: $functions_to_long }
   - match: { esql.functions.coalesce: $functions_coalesce }
   - gt: { esql.functions.categorize: $functions_categorize }
-  # - length: {esql.functions: 151} # check the "sister" test above for a likely update to the same esql.functions length check
+
+  # There's one of these per function but that's a ton of things to check. So we just spot check that a few exist.
+  - not_exists: esql.functions.delay
+  - not_exists: esql.functions.idelta
+  - exists: esql.functions.mv_sum
+  - exists: esql.functions.to_dateperiod
 
 ---
 took: