types/logger: fix rateFree interaction with verbosity prefixes

We log lines like this:

c.logf("[v1] magicsock: disco: %v->%v (%v, %v) sent %v", c.discoShort, dstDisco.ShortString(), dstKey.ShortString(), derpStr(dst.String()), disco.MessageSummary(m))

The leading [v1] causes it to get unintentionally rate limited.
Until we have a proper fix, work around it.

Fixes #1216

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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