[prototype] sql,tracing: per-stmt probabilistic trace sampling

[irfansharif]

Prototyping something that could help #80666. I'm hoping we can get to
the following ability:

  Pick a stmt fingerprint, declare a sampling probability which controls
  when verbose tracing is enabled for it, and a latency threshold for
  which a trace is persisted/logged/etc. Maybe even a count of how many
  traces I actually want recorded? Some of these controls could be in the
  UI, or traces made visible in the UI, but I’d happily take a set of
  dubious cluster settings or table writes to install these
  to-be-traced-with-some-probability fingerprints, or just vanilla log
  outputs. With a given stmt rate (say 1000/s) and a given percentile I’m
  trying to capture (say p99.9), we have 1 stmt/s in the 99.9th
  percentile. I need to make this math less handwavey, but I should be
  able to set a sampling probability P such that with high likelihood
  (>95%) we capture at least one trace captured over the next S seconds. The
  sampling rate lets you control how the overhead you’re introducing for
  those statements in aggregate, which if dialed up higher lets you
  lower S. You might want to do such a thing for infrequently executed
  statements.

Still not sure how to figure out what statement to trace (we don't
capture per-stmt %-iles -- too expensive? Maybe we will in some form
after #82473?). And also not sure what we can do better than just
logging with captured traces (stash them under /tracez somehow?) or in
some system table capped in # of rows + with row-level TTL to evict
captures + make room for new, more recent captures? Anyway, this first
stab does the simplest thing: ask for all input through shoddy cluster
settings and just log stuff. A quick and dirty attempt to try and catch
an outlier execution for the history table write in TPC-C. Didn't see an
overall increase in p99s as a result of the sampling + took about 10s to
capture this, showing it was due to latch waits.

> SELECT
  encode(fingerprint_id, 'hex'),
  (statistics -> 'statistics' ->> 'cnt')::INT AS count,
  metadata ->> 'query' AS query
FROM system.statement_statistics
ORDER BY COUNT DESC limit 10;

       encode    | count |                                             query
-----------------+-------+--------------------------------------------------------------------------------------------------------------------
...
4e4214880f87d799 |  2680 | INSERT INTO history(h_c_id, h_c_d_id, h_c_w_id, h_d_id, h_w_id, h_amount, h_date, h_data) VALUES ($1, $2, __more6__)

> SET CLUSTER SETTING trace.fingerprint = '4e4214880f87d799';
> SET CLUSTER SETTING trace.fingerprint.probability = 0.01;
> SET CLUSTER SETTING trace.fingerprint.threshold = '90ms';

 0.521ms      0.005ms    event:kv/kvserver/concurrency/concurrency_manager.go:260 [n1,s1,r105/1:/Table/109/1/{90/7/2…-105/10…}] acquiring latches›
 0.532ms      0.011ms    event:kv/kvserver/spanlatch/manager.go:532 [n1,s1,r105/1:/Table/109/1/{90/7/2…-105/10…}] waiting to acquire read latch /Table/109/1/105/3/519/0@0,0, held by write latch /Table/109/1/105/3/519/0@1654849354.483984000,0›
99.838ms     99.306ms    event:kv/kvserver/concurrency/concurrency_manager.go:301 [n1,s1,r105/1:/Table/109/1/{90/7/2…-105/10…}] scanning lock table for conflicting locks›
99.851ms      0.013ms    event:kv/kvserver/replica_read.go:251 [n1,s1,r105/1:/Table/109/1/{90/7/2…-105/10…}] executing read-only batch›

Release note: None

[cockroach-teamcity]

This change is 

[irfansharif]

Might keep this janky form around to get a backportable form of #82750 for 21.2 & 22.1.

[irfansharif]

Ugh, I lost the commit history somewhere and force pushed to the wrong branch.