bootstrap: reject routes that conflict with protected paths

Handle/HandleFunc let callers register any pattern, including the metrics
path or root, which the mux would otherwise reject with a raw ServeMux
panic at startup. Validate against the protected set (--web.telemetry-path
and /) in newServer and return a normal error instead.

Signed-off-by: Nicolas Takashi <nicolas.tcs@hotmail.com>