FR: Support hostname sysvar

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

Because tidb server instances are usually configured behind a load balancer, it's not usually possible to tell which server you are connected to.

If there are issues, you may want to record the identity of the server. Or you may even want to know it to evaluate load balancing.

In the case of automated tools (like a logical backup tool), they can use the hostname as the default for the backup file name. Or a status tool might print the hostname so you can confirm which server you are connected to.

Describe the feature you'd like:

With MySQL you can do this by reading the hostname sysvar:

mysql [localhost] {msandbox} ((none)) > SELECT @@hostname;
+------------+
| @@hostname |
+------------+
| ryzen      |
+------------+
1 row in set (0.00 sec)

In TiDB, the hostname sysvar is not supported yet. It looks like a trivial enhancement, I'd like to request that it is:

mysql> SELECT @@hostname;
+------------+
| @@hostname |
+------------+
| localhost  |
+------------+
1 row in set (0.00 sec)

./sessionctx/variable/sysvar.go:

  {ScopeNone, "innodb_checksums", "ON"},
  {ScopeNone, "hostname", "localhost"},
  {ScopeGlobal | ScopeSession, "auto_increment_offset", "1"},

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

Compatibility Feature.