make "combine streams" example easier to understand

I spent a fair amount of time trying to grasp how to merge 2 streams, and this terse example frustrated me somewhat.

Copy-pasting the previous code didn't seem to do anything. After adding the `(streams ...)` block, you can now see something when you query the index. Furthermore, I also suggest indexing the 2 source streams as well as the calculated rate with a different service name. This makes the example a bit longer, but what you see in the dash then makes a lot more sense and helps understanding imho.

It can also be that I completely misunderstood the whole thing... Let me know, I won't be disappointed if you don't merge it ;)