sql: Add SHOW LOCALITY command.

[rohany]

Fixes #38438.

SHOW LOCALITY returns the locality of the current node.

Release note (sql change): Add a SHOW LOCALITY command in sql.

[cockroach-teamcity]

This change is 

[rohany]

./cockroach start --listen-addr=localhost --insecure --locality=region=us-east1
------
➜  cockroach git:(show-locality) ✗ ./cockroach sql --insecure
# Welcome to the cockroach SQL interface.
# All statements must be terminated by a semicolon.
# To exit: CTRL + D.
#
# Server version: CockroachDB CCL v19.2.0-alpha.20190606-964-ge9c579a369-dirty (x86_64-apple-darwin18.6.0, built 2019/07/23 19:27:01, go1.12.5) (same version as client)
# Cluster ID: 1be30eaf-3f36-4ae6-a2e7-867e4fce7410
# Organization: Cockroach Labs - Production Testing
#
# Enter \? for a brief introduction.
#
root@:26257/defaultdb> show locality;
     locality
+-----------------+
  region=us-east1
(1 row)

Time: 419µs

root@:26257/defaultdb>

[solongordon]

• Rather than making this a query, I think it would better if locality was just a new variable like node_id is. See here:

  cockroach/pkg/sql/vars.go

  Lines 520 to 525 in cee171c

  	// CockroachDB extension.
  	`node_id`: {
  	Get: func(evalCtx *extendedEvalContext) string {
  	return fmt.Sprintf("%d", evalCtx.NodeID)
  	},
  	},

  The implementation should look very similar actually since I think locality is stored on the evalCtx.
• What do we want this to show if there is no locality? Right now it's blank, which is maybe fine or maybe we would prefer "none" or something? I guess we can make a note to ask @awoods187 when he gets back.
• For commits like this with user-facing changes, you should fill out the "Release note" part of the commit message. That will be used by the docs team when they generate release notes, like this: https://www.cockroachlabs.com/docs/releases/v19.2.0-alpha.20190701.html

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @awoods187 and @solongordon)

[solongordon]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @awoods187, @rohany, and @solongordon)

---

pkg/sql/vars.go, line 516 at r1 (raw file):

				return evalCtx.Locality.String()
			}
			return "No locality set."

I changed my mind, I think this is kind of weird. :) Let's just stick with the blank value.

[solongordon]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @awoods187 and @rohany)

[rohany]

bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)