Behaviour of init functions has been centralised in `collector/init.go`,
and can be called during exporter startup. This allows the exporter to
control the timing of collector initialisation, rather than relying on
the import & `init()` method.
This should reduce unexpected behaviour arising from the use of
`init()`, such as #551.
Signed-off-by: Ben Reedy <breed808@breed808.com>
