test(telemetry): assert the nodestore_state labels and pin writer depth

The 22 metric label values on the nodestore_state gauge were asserted
nowhere, and the depthSum assertions could not tell the real
fetch_add(depth) from a fetch_add(1) that would silently zero every
derived queueing time.

Make the four observe* helpers and their ObserveFn sink public rather
than private, so the seam the header already claimed is actually
reachable. All four are static and read only their arguments, so this
exposes no object state; a friend declaration would have granted access
to every private member instead. The assertions live in the existing
Beast nodestore suite because src/test compiles into xrpld, which
contains MetricsRegistry.cpp, while xrpl_tests deliberately does not
when telemetry is enabled.

Each helper now has its exact emitted label set asserted, so a typo in
any literal fails instead of silently producing a disjoint series, and
each derived mean is asserted ABSENT on a fresh store -- a refactor to
value_or(0) would draw a believable flat zero on a latency axis and
otherwise pass everything.

Replace the concurrent write-stats test with one that forces genuine
overlap through a latch. NuDB holds one global mutex for the whole
insert and doInsert reads the depth before entering it, so a blocked
thread records a depth of at least 2; asserting depthSum strictly
exceeds insertCount therefore cannot be satisfied by a constant 1. The
old bounds admitted that bug at their floor.

Also: cover the std::nullopt branch on the two backends that exist in
every build, bound the store-duration accumulator by the wall clock,
drop four assertions that cannot fail, and correct two comments that
claimed coverage the tests do not have -- the duplicate-key test is not
the throwing path, because nudb reports key_exists without throwing,
and no test drives NodeStoreScheduler::onFetch.

.github/scripts/levelization/results/ordering.txt

@@ -138,7 +138,9 @@ test.nodestore > test.jtx
 test.nodestore > test.unit_test
 test.nodestore > xrpl.basics
 test.nodestore > xrpl.config
+test.nodestore > xrpld.app
 test.nodestore > xrpld.core
+test.nodestore > xrpld.telemetry
 test.nodestore > xrpl.nodestore
 test.nodestore > xrpl.rdb
 test.overlay > test.jtx