Some varnish metric names have changed in 4.x

[wang-arthur]

In newer versions of Varnish, stats that previously did not include types now include the type MAIN. For example,

    <stat>
        <name>backend_req</name>
        <value>0</value>
        <flag>a</flag>
        <description>Backend requests made</description>
    </stat>

is now

    <stat>
        <type>MAIN</type>
        <name>backend_req</name>
        <value>0</value>
        <flag>a</flag>
        <description>Backend requests made</description>
    </stat>

Because we prefix varnish metric names with the stats' type, metrics like these are now appearing in Datadog as varnish.MAIN.<metric_name> instead of varnish.<metric_name>, and the Varnish dashboard, which uses metrics in the latter format, appear as blank.

For consistency, we should continue reporting metrics in the format varnish.<metric_name>.