Merge apps by label

[messiaen]

In our system we have several thrift services, which have a cpu and gpu implementation. This means we need to deploy and app for the cpu implementation and another app for the gpu implementation. We would like that fact to be hidden from clients (i.e. be able to access the cpu and gpu implementations of a service at the same address).

For example if we have the following apps running in mesos/marathon, which both implement the same api

my-service-cpu
my-service-gpu

We would like for clients to be able to access both implementations at the same address (e.g nixy.marathon.mesos:12345)

Our solution was to specify a service name (e.g. my-service) via a label, and then add a method to the Config struct (MergeAppsByLabel(label)) which merges apps based on the given label. The idea is that users could call that method in their templates instead of just .Apps so that the different implementations could all have a single upstream definition.

- This allows for example, multiple implementations of a
  service to be accessed using the same address even though they are
  running as different apps within mesos/marathon.
- Users specify a label which can be used as the service name
  across multiple apps, and then call MergeAppsByLabel within their
  template.
- When the apps are merged Labels from each original app are added
  to each Task from that original app. This is necessary if you want
  to have implementation specific labels.  For example, if one
  implementation is faster, we could route more traffic there.

[martensson]

Hi @messiaen! Thanks for the added feature, its an interesting case you got there. Could you please also add the "MergeAppsByLabel" to the documentation (README) so others might know/find how to use it? Will then merge with no problem!

[messiaen]

@martensson Great! I added some docs to the README, and added and example nginx template.

[martensson]

Thanks for the doc and example config @messiaen! 😄