util/expvarx: add a time and concurrency limiting expvar.Func wrapper

expvarx.SafeFunc wraps an expvar.Func with a time limit. On reaching the
time limit, calls to Value return nil, and no new concurrent calls to
the underlying expvar.Func will be started until the call completes.

Updates tailscale/corp#16999
Signed-off-by: James Tucker <james@tailscale.com>
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