support sharded graphite using metrics/expand instead of index.json

[nikicat]

[Dieterbe]

looks like we should use graphite-project/graphite-web#329

[Dieterbe]

so graphite-project/graphite-web#550 was never merged,
but graphite-project/graphite-web#329 is

I think the only thing we should do is request index.json?cluster=1 instead of just index.json

[deejay1]

index.json?cluster=1 is not supported in all infrastructure types, IMHO there should be a fallback if index.json returns [ ] for discovery using metrics/find or metrics/expand as at least the first one works at our setup (I haven't checked the second one yet)

[Dieterbe]

can you explain in which cases index.json?cluster=1 works and in which it doesn't?