ConsistentHashing: provide use_hostname_for_hashing for eds based discovery.

[fcantournet]

To achieve stable consistent hashing over endpoints in ringhash/maglev when using EDS we need something similar to use_hostname_for_hashing.

In e.g: kubernetes using statefulsets you have endpoints with a stable identity, but whose IP might change during rollouts.

If you use use_hostname_for_hashing and STRICT_DNS with the records for each endpoint you can achieve stable consistent hashing during rollouts, with minimal re-shuffling.

Can we make use_hostname_for_hashing work with EDS by using the hostname field of Endpoint ?

[RyanTheOptimist]

cc: @htuch @mattklein123

It looks like the eds cluster is currently unowned according to CODEOWNERS.md:
/*/extensions/clusters/eds/ @UNOWNED @UNOWNED
But perhaps that just needs to be updated?

[htuch]

@adisuissa

[adisuissa]

I think that a pre-req to this is to get EDS working with non-ip hostnames, no?
This was requested quite a long time ago (#4290) but I don't think it gained enough momentum to be worked on.

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