gopls/internal/telemetry: skip the telemetry test on Plan 9

The telemetry test relies on the counters from
golang.org/x/telemetry, which are stored in
memory-mapped files.

Plan 9 does not support mmap, so the counters are
never persisted and always read back as zero, like
on js/wasm and wasip1, which are already excluded.

Add plan9 to the build constraint.

Change-Id: I356f9d0115ace200de8c4ffa3a192ca18babeaba
Reviewed-on: https://go-review.googlesource.com/c/tools/+/794821
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David du Colombier <0intro@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Richard Miller <millerresearch@gmail.com>

gopls/internal/telemetry/telemetry_test.go

diff --git a/gopls/internal/telemetry/telemetry_test.go b/gopls/internal/telemetry/telemetry_test.go
index a8cf2b8..a5e7c6f 100644
--- a/gopls/internal/telemetry/telemetry_test.go
+++ b/gopls/internal/telemetry/telemetry_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !openbsd && !js && !wasip1 && !solaris && !android && !386
+//go:build !openbsd && !js && !wasip1 && !solaris && !android && !386 && !plan9
 
 package telemetry_test