As of this writing, `system.query_log` is one of the largest clickhouse tables.
From dogfood:

```
┌─database─┬─table──────────────────────────┬─compressed─┬─uncompressed─┬───────rows─┐
│ system   │ query_log                      │ 17.11 GiB  │ 39.49 GiB    │   24942352 │
│ oximeter │ measurements_cumulativeu64     │ 15.52 GiB  │ 91.91 GiB    │ 1576299396 │
│ oximeter │ measurements_f32               │ 14.52 GiB  │ 85.23 GiB    │ 1849126423 │
│ oximeter │ measurements_histogramu64      │ 2.76 GiB   │ 117.37 GiB   │   78425393 │
│ system   │ metric_log                     │ 742.29 MiB │ 12.84 GiB    │    2543307 │
└──────────┴────────────────────────────────┴────────────┴──────────────┴────────────┘
```

Most of this table's disk use is attributable to the `query` column, and most
of that usage comes from the very long measurement queries run by oximeter,
which take the form of:

```
SELECT * FROM oximeter.measurements_* WHERE timeseries_key IN (...)
```

The `IN` clause can include thousands of keys, and a single oximeter query can
run multiple clickhouse queries of this form. This takes up a lot of space in
the query log.

This patch truncates long `IN (...)` clauses. These aren't operationally
useful, and shortening them shrinks `system.query_log` by about 80% in testing.