[ES|QL] Add support for ByteLength scalar function

Author: GalLalouche

docs/reference/esql/functions/description/byte_length.asciidoc

@@ -47,6 +47,27 @@ BIT_LENGTH(string_exp) <1>
 include-tagged::{sql-specs}/docs/docs.csv-spec[stringBitLength]
 --------------------------------------------------
 
+[[sql-functions-string-byte-length]]
+==== `BYTE_LENGTH`
+
+.Synopsis:
+[source, sql]
+--------------------------------------------------
+BYTE_LENGTH(string_exp) <1>
+--------------------------------------------------
+*Input*:
+
+<1> string expression. If `null`, the function returns `null`.
+
+*Output*: integer
+
+*Description*: Returns the length in bytes of the `string_exp` input expression.
+
+[source, sql]
+--------------------------------------------------
+include-tagged::{sql-specs}/docs/docs.csv-spec[stringByteLength]
+--------------------------------------------------
+
 [[sql-functions-string-char]]
 ==== `CHAR`