bootstrap: allow exporters to add routes and a metrics path envar

The bootstrap package builds its own mux, so an exporter that serves more
than /metrics and the landing page cannot adopt it. postgres_exporter, for
example, serves the multi-target /probe endpoint and the net/http/pprof
handlers, and node_exporter-style adoption would silently drop both.

Give Bootstrap a Handle/HandleFunc pair. Routes registered from the metrics
handler factory are applied to the mux next to the metrics endpoint, which
keeps them constructible from state that only exists after flags are
parsed, such as the logger and the loaded config.

Exporters that shipped --web.telemetry-path with an environment variable
also cannot keep that behavior once bootstrap owns the flag, so allow the
envar name to be configured.

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