enable .tpc author to declare partition index in a source block

[judell]

There are cases where the thing you'd want to use for a partition index does not appear in the logs, so the plugin won't be able to know what a meaningful index would be, but the user would. With the nginx plugin, for example, I played games with tp_source_location to derive an index for two different servers but that seems fragile, it would be more natural for me as the user to just declare server1, server2, etc. on a per-source basis.