sql: lower default sampling rate to 1%

We arrived at the previous default rate of 10% back in cockroachdb#59379. This was
back when we were creating real tracing spans for all statements, and
for sampled statements, we were propagating additional stats payloads.
Consequently what cockroachdb#59379 ended up measuring (and finding the overhead
acceptable) for was the performance hit we would incur for propagating
stats payloads for statements already using real tracing spans.

Since then, the landscape has changed. Notably we introduced cockroachdb#61777,
which made it so that we were only using real tracing spans for sampled
statements. This was done after performance analysis in cockroachdb#59424 showed
that the use of real tracing spans in all statements resulted in
tremendous overhead, for no real benefit.

What this now leaves us with is a sampling rate that was tuned by only
considering the stats payload overhead. What we want now is to also
consider the overhead of using real tracing spans for sampled
statements, vs. not. Doing this analysis gives us a very different
picture for what the default rate should be.

---

To find out what the overhead for sampled statements are currently, we
experimented with kv95/enc=false/nodes=1/cpu=32. It's a simple
benchmark that does little more than one-off statements, so should give
us a concise picture of the sampling overhead. We ran six experiments
in total (each corresponding to a pair of read+write rows), done in
groups of three (each group corresponding to a table below). Each
run in turn is comprised of 10 iterations of kv95, and what's varied
between each run is the default sampling rate. We pin a sampling rate of
0.0 as the baseline that effectively switches off sampling entirely (and
tracing), and measure the throughput degradation as we vary the sampling
rate.

                          ops/sec            ops/sec
    --------------------|------------------|------------------
    rate   op      grp  | median    diff   | mean      diff
    --------------------|------------------|------------------
    0.00 / read  / #1   | 69817.90         | 69406.37
    0.01 / read  / #1   | 69300.35  -0.74% | 68717.23  -0.99%
    0.10 / read  / #1   | 67743.35  -2.97% | 67601.81  -2.60%
    0.00 / write / #1   |  3672.55         |  3653.63
    0.01 / write / #1   |  3647.65  -0.68% |  3615.90  -1.03%
    0.10 / write / #1   |  3567.20  -2.87% |  3558.90  -2.59%

                          ops/sec            ops/sec
    --------------------|------------------|------------------
    rate   op      grp  | median    diff   | mean      diff
    --------------------|------------------|------------------
    0.00 / read  / #2   | 69440.80          68893.24
    0.01 / read  / #2   | 69481.55  +0.06%  69463.13  +0.82% (probably in the noise margin)
    0.10 / read  / #2   | 67841.80  -2.30%  66992.55  -2.76%
    0.00 / write / #2   |  3652.45           3625.24
    0.01 / write / #2   |  3657.55  -0.14%   3654.34  +0.80%
    0.10 / write / #2   |  3570.75  -2.24%   3526.04  -2.74%

The results above suggest that the current default rate of 10% is too
high, and a 1% rate is much more acceptable.

---

The fact that the cost of sampling is largely dominated by tracing is
extremely unfortunate. We have ideas for how that can be improved
(prototyped in cockroachdb#62227), but they're much too invasive to backport to
21.1. It's unfortunate that we only discovered the overhead this late in
the development cycle. It was due to two major reasons:
- cockroachdb#59992 landed late in the cycle, and enabled tracing for realsies (by
  propagating real tracing spans across rpc boundaries). We had done
  sanity checking for the tracing overhead before this point, but failed
  to realize that cockroachdb#59992 would merit re-analysis.
- The test that alerted us to the degradation (tpccbench) had be
  persistently failing for a myriad of other reasons, so we didn't learn
  until too late that tracing was the latest offendor. tpccbench also
  doesn't deal with VM overload well (something cockroachdb#62361 hopes to
  address), and after tracing was enabled for realsies, this was the
  dominant failure mode. This resulted in perf data not making it's way
  to roachperf, which further hid possible indicators we had a major
  regression on our hands. We also didn't have a healthy process looking
  at roachperf on a continual basis, something we're looking to rectify
  going forward. We would've picked up on this regression had we been
  closely monitoring the kv95 charts.

Release note: None

pkg/sql/instrumentation.go

@@ -38,7 +38,7 @@ import (
 var collectTxnStatsSampleRate = settings.RegisterFloatSetting(
  "sql.txn_stats.sample_rate",
  "the probability that a given transaction will collect execution statistics (displayed in the DB Console)",
- 0.1,
+ 0.01,
  func(f float64) error {
  if f < 0 || f > 1 {
  return errors.New("value must be between 0 and 1 inclusive")