ci: use a count-based fuzztime limit to dodge a Go toolchain race

Duration-based -fuzztime (e.g. 5s) hits a known unfixed race in
internal/fuzz where the context deadline used to stop workers can
leak past the code that's supposed to suppress it as a normal stop
signal, failing the run with a bare "context deadline exceeded" and
no crasher input (golang/go#75804). Confirmed against upstream repro
output and the Go team's own workaround: reproduces regardless of
fuzztime length (seen from 3s to 10+ minutes upstream), and the fix
(CL 804900) only just landed in release-branch.go1.27, not yet in any
released stable/oldstable toolchain.

Switching to a count-based limit (Nx) avoids the wall-clock deadline
context entirely, sidestepping the race rather than gambling that a
longer duration makes it less likely.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: kevinburkesegment

.github/workflows/go.yml

@@ -37,4 +37,4 @@ jobs:
         run: go test -trimpath -race ./...
 
       - name: Run Fuzz Tests
-        run: go test -fuzz=FuzzAppendSanitizedMetricName -fuzztime=5s -parallel=2 ./datadog
+        run: go test -fuzz=FuzzAppendSanitizedMetricName -fuzztime=1000000x -parallel=2 ./datadog