Add a new "ipnbus" build feature tag so the watch-ipn-bus LocalAPI
endpoint can be independently controlled, rather than being gated
behind HasDebug || HasServe. Minimal/embedded builds that omit both
debug and serve were getting 404s on watch-ipn-bus, breaking
"tailscale up --authkey=..." and other CLI flows that depend on
WatchIPNBus.
In the CLI, check buildfeatures.HasIPNBus before attempting to watch
the IPN bus in "tailscale up"/"tailscale login", and exit early with
an informational message when the feature is omitted.
Also add the missing NewCounterFunc stub to clientmetric/omit.go,
which caused compilation errors when building with
ts_omit_clientmetrics and netstack enabled.
Fixes#19240
Change-Id: I2e3c69a72fc50fa02542b91b8a54859618a463d1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
1b5b43787c