Change default of histogram_type to JSON_HB

mysql-test/main/mysqld--help.result

@@ -1632,7 +1632,7 @@ gtid-pos-auto-engines
 gtid-strict-mode FALSE
 help TRUE
 histogram-size 254
-histogram-type DOUBLE_PREC_HB
+histogram-type JSON_HB
 host-cache-size 279
 idle-readonly-transaction-timeout 0
 idle-transaction-timeout 0

mysql-test/suite/sys_vars/r/histogram_type_basic.result

@@ -10,7 +10,7 @@ SET @@global.histogram_type = 1;
 SET @@global.histogram_type = DEFAULT;
 SELECT @@global.histogram_type;
 @@global.histogram_type
-DOUBLE_PREC_HB
+JSON_HB
 SET @@global.histogram_type = 0;
 SELECT @@global.histogram_type;
 @@global.histogram_type

sql/sys_vars.cc

@@ -6670,7 +6670,7 @@ static Sys_var_enum Sys_histogram_type(
        "DOUBLE_PREC_HB - double precision height-balanced, "
        "JSON_HB - height-balanced, stored as JSON.",
        SESSION_VAR(histogram_type), CMD_LINE(REQUIRED_ARG),
-       histogram_types, DEFAULT(1));
+       histogram_types, DEFAULT(2));
 
 static Sys_var_mybool Sys_no_thread_alarm(
        "debug_no_thread_alarm",