[roll third_party/pigweed] pw_allocator: Fix metric disabling

This CL corrects the previously inverted condition on
`pw_allocator_COLLECT_METRICS`.

It also makes the `MetricsStub` not derive from `metric::Group`, so as
to avoid pulling in metric code unnecessarily.

Finally, the fixes above revealed pw_multibuf's test utilities were not
using `AllocatorMetricProxyForTest` when it intended to.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/185026

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 752d18a519176d4..9ea4f88a8b21a90
Roller-URL: https://ci.chromium.org/b/8761697786194652385
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib83bf0c4db1a187d75d92c97b12d75eccf7c9b86
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/185151
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 752d18a..9ea4f88 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 752d18a519176d462bc61428bb4d2e1b617b3d6f
+Subproject commit 9ea4f88a8b21a909b41462b2420bbbe8bfa97187