When more than one prefix matches the same metric descriptor, this will
throw the error "collected metric xxx was collected before with the
same name and label values".

For example, using the metric type prefixes
  foo.googleapis.com/bar (a prefix)
and
  foo.googleapis.com/bar/baz (a metric)
will result in an error because both match the metric
  foo.googleapis.com/bar/baz.

Further, using the metric type prefixes
  foo.googleapis.com/bar/baz (a metric)
and
  foo.googleapis.com/bar/baz_count (a metric)
will result in an error because both match the metric
  foo.googleapis.com/bar/baz_count.

While the first pitfall could be expected by the user, the latter will
come as a complete surprise to anyone who is not aware that
stackdriver-exporter internally uses an MQL query in the form of
  metric.type = starts_with("<prefix>")
to filter the metrics.

Avoid this by sanitizing the provided metric type prefixes in the
following way:
- Drop any duplicate prefixes
- Sort the prefixes (required by the next step)
- Drop any prefixes that start with another prefix present in the input

Signed-off-by: Edwin Mackenzie-Owen <edwin.mowen@gmail.com>

In alphanumerically sorted list of strings,
abcdef
will never come before
abc.

Signed-off-by: Edwin Mackenzie-Owen <edwin.mowen@gmail.com>

Signed-off-by: Edwin Mackenzie-Owen <edwin.mowen@gmail.com>