release 21.2: sql: record index usage stats in index joins

[xinhaoz]

Backport 1/1 commits from #78333.

/cc @cockroachdb/release

---

Fixes #76173

Previously, the construction of index joins skipped the
recording of the primary key for index usage stats.
This commit records the use of the primary key if the
statement is not of the EXPLAIN variety.

Release note (bug fix): Index usage stats are now properly
captured for index joins.

Release justification: bug fix

---

Example: We should expect to see a read from the pkey after an index join.

root@:26257/defaultdb> CREATE TABLE test (c1 INT, c2 INT, c3 INT, INDEX(c1, c2));
CREATE TABLE


Time: 120ms total (execution 120ms / network 0ms)

root@:26257/defaultdb> EXPLAIN select c3 from TEST WHERE c2 > 1 AND c1 = 3;
                                              info
------------------------------------------------------------------------------------------------
  distribution: local
  vectorized: true

  • index join
  │ estimated row count: 1
  │ table: test@test_pkey
  │
  └── • scan
        estimated row count: 1 (100% of the table; stats collected 49 seconds ago)
        table: test@test_c1_c2_idx
        spans: [/3/2 - /3]

  index recommendations: 1
  1. type: index replacement
     SQL commands: CREATE INDEX ON test (c1, c2) STORING (c3); DROP INDEX test@test_c1_c2_idx;
(15 rows)


Time: 1ms total (execution 1ms / network 0ms)

root@:26257/defaultdb> select c3 from TEST WHERE c2 > 1 AND c1 = 3;
  c3
------
(0 rows)


Time: 1ms total (execution 1ms / network 0ms)

root@:26257/defaultdb> select index_name, index_type, total_reads from crdb_internal.index_usage_statistics AS us JOIN crdb_internal.table_indexes ti ON us.index_id = ti.index_id AND us.table_id = ti.descriptor_id ORDER B

[cockroach-teamcity]

This change is 

[maryliag]

Reviewable status: complete! 1 of 0 LGTMs obtained