Added note on UTF-8, added comment on telemetry

Author: Gal (Windows)

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

@@ -40,6 +40,7 @@ public class BitLength extends UnaryScalarFunction {
     @FunctionInfo(
         returnType = "integer",
         description = "Returns the bit length of a string.",
+        note = "All strings are in UTF-8, so a single character can use multiple bytes.",
         examples = @Example(file = "docs", tag = "bitLength")
     )
     public BitLength(

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

@@ -37,6 +37,7 @@ public class ByteLength extends UnaryScalarFunction {
     @FunctionInfo(
         returnType = "integer",
         description = "Returns the byte length of a string.",
+        note = "All strings are in UTF-8, so a single character can use multiple bytes.",
         examples = @Example(file = "eval", tag = "byteLength")
     )
     public ByteLength(

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

@@ -34,6 +34,7 @@ public class Length extends UnaryScalarFunction {
     @FunctionInfo(
         returnType = "integer",
         description = "Returns the character length of a string.",
+        note = "All strings are in UTF-8, so a single character can use multiple bytes.",
         examples = @Example(file = "eval", tag = "length")
     )
     public Length(

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

@@ -30,7 +30,7 @@ setup:
         - method: POST
           path: /_query
           parameters: []
-          capabilities: [ snapshot_test_for_telemetry, fn_bit_length ]
+          capabilities: [ snapshot_test_for_telemetry, fn_byte_length ]
       reason: "Test that should only be executed on snapshot versions"
 
   - do: {xpack.usage: {}}
@@ -91,6 +91,7 @@ setup:
   - match: {esql.functions.cos: $functions_cos}
   - gt: {esql.functions.to_long: $functions_to_long}
   - match: {esql.functions.coalesce: $functions_coalesce}
+  # Testing for the entire function set isn't feasbile, so we just check that we return the correct count as an approximation.
   - length: {esql.functions: 120} # check the "sister" test below for a likely update to the same esql.functions length check
 
 ---
@@ -101,7 +102,7 @@ setup:
         - method: POST
           path: /_query
           parameters: []
-          capabilities: [ non_snapshot_test_for_telemetry, fn_bit_length ]
+          capabilities: [ non_snapshot_test_for_telemetry, fn_byte_length ]
       reason: "Test that should only be executed on release versions"
 
   - do: {xpack.usage: {}}