capitalize sql keywords (#4367)

* capitalize sql keywords

* capitalize more sql keywords

* Update metrics-schema.md

Co-authored-by: Charlotte Liu <37295236+CharLotteiu@users.noreply.github.com>

* capitalize L188 keywords

Co-authored-by: Ran <huangran@pingcap.com>
Co-authored-by: Charlotte Liu <37295236+CharLotteiu@users.noreply.github.com>

metrics-schema.md

@@ -11,7 +11,7 @@ The `METRICS_SCHEMA` is a set of views on top of TiDB metrics that are stored in
 {{< copyable "sql" >}}
 
 ```sql
-use metrics_schema;
+USE metrics_schema;
 SELECT * FROM uptime;
 SELECT * FROM information_schema.metrics_tables WHERE table_name='uptime'\G
 ```
@@ -43,7 +43,7 @@ TABLE_NAME: uptime
 {{< copyable "sql" >}}
 
 ```sql
-show tables;
+SHOW TABLES;
 ```
 
 ```sql
@@ -105,7 +105,7 @@ Query the information related to the `tidb_query_duration` table on `information
 {{< copyable "sql" >}}
 
 ```sql
-select * from information_schema.metrics_tables where table_name='tidb_query_duration';
+SELECT * FROM information_schema.metrics_tables WHERE table_name='tidb_query_duration';
 ```
 
 ```sql
@@ -129,7 +129,7 @@ To query the schema of the `tidb_query_duration` table, execute the following st
 {{< copyable "sql" >}}
 
 ```sql
-show create table metrics_schema.tidb_query_duration;
+SHOW CREATE TABLE metrics_schema.tidb_query_duration;
 ```
 
 ```sql
@@ -156,7 +156,7 @@ The following statement queries the P99 time within the range of [`2020-03-25 23
 {{< copyable "sql" >}}
 
 ```sql
-select * from metrics_schema.tidb_query_duration where value is not null and time>='2020-03-25 23:40:00' and time <= '2020-03-25 23:42:00' and quantile=0.99;
+SELECT * FROM metrics_schema.tidb_query_duration WHERE value is not null AND time>='2020-03-25 23:40:00' AND time <= '2020-03-25 23:42:00' AND quantile=0.99;
 ```
 
 ```sql
@@ -185,7 +185,7 @@ To view the execution plan of the statement above, execute the following stateme
 {{< copyable "sql" >}}
 
 ```sql
-desc select * from metrics_schema.tidb_query_duration where value is not null and time>='2020-03-25 23:40:00' and time <= '2020-03-25 23:42:00' and quantile=0.99;
+DESC SELECT * FROM metrics_schema.tidb_query_duration WHERE value is not null AND time>='2020-03-25 23:40:00' AND time <= '2020-03-25 23:42:00' AND quantile=0.99;
 ```
 
 ```sql