Report used, max and percent values for conntrack consumption

Currently the conntrack module only emits absolute values
for the conntrack. This change reports two additional values
which can help prevent additional computing on the aggregation
layer.

One thing to note is the fact that `conntrack/conntrack` now
becomes `conntrack/conntrack-used`. If necessary, I can revert
`conntrack/conntrack-used` to `conntrack/conntrack`.