bootstrap: allow overriding the root route, keep the metrics path reserved

Per review on prometheus/exporter-toolkit#430: reserving / blocks exporters
that already serve something there (Prometheus itself redirects / to
/query). A caller-registered route at / now takes precedence over the
generated landing page. The metrics path stays reserved, since silently
losing the scrape endpoint to a route collision is a much worse failure
mode than losing the landing page.

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