runtime: don't re-read metrics before check in TestReadMetricsSched

The two remaining popular flakes in TestReadMetricsSched are with
waiting and not-in-go goroutines. I believe the reason for both of these
is pollution due to other goroutines in the test binary, and we can be a
little bit more robust to them.

In particular, both of these tests spin until there's a particular
condition met in the metrics. Then they both immediately re-read the
metrics. The metrics being checked are not guaranteed to stay that way
in the re-read. For example, for the waiting case, it's possible for
>1000 to be waiting, but then some leftover goroutines from a previous
test wake up off of some condition, causing the number to dip again on
the re-read. This is supported by the fact that in some of these
failures, the test takes very little time to execute (the condition that
there are at least 1000 waiting goroutines is almost immediately
satisfied) but it still fails (the re-read causes us to notice that there
are <1000 waiting goroutines).

This change makes the test more robust by not performing this re-read.
This means more possible false negatives, but the error cases we're
looking for (bad metrics) should still show up with some reasonable
probability when something *is* truly wrong.

For #75049.

Change-Id: Idc3a8030bed8da51f24322efe15f3ff13da05623
Reviewed-on: https://go-review.googlesource.com/c/go/+/705875
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mknyszek

src/runtime/metrics_test.go

@@ -1760,8 +1760,6 @@ func TestReadMetricsSched(t *testing.T) {
 			metrics.Read(s[:])
 			return s[notInGo].Value.Uint64() >= count
 		})
-
-		metrics.Read(s[:])
 		logMetrics(t, s[:])
 		check(t, &s[notInGo], count, count+generalSlack)
 
@@ -1782,8 +1780,6 @@ func TestReadMetricsSched(t *testing.T) {
 			metrics.Read(s[:])
 			return s[waiting].Value.Uint64() >= waitingCount
 		})
-
-		metrics.Read(s[:])
 		logMetrics(t, s[:])
 		check(t, &s[waiting], waitingCount, waitingCount+waitingSlack)