store: Add Memcached Index Cache mixin

[kakkoyun]

This PR adds withMemcachedIndexCache to Thanos Store to use memcached as index cache.
Enables posting compression.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Adds withMemcachedIndexCache mixin.
• Enables posting compression.
• Added flags from Thanos Store:
  • --experimental.enable-index-cache-postings-compression
  • --index-cache.config=${CONFIG}'

Verification

• make generate

[brancz]

DCO missing, otherwise looks really awesome!

[kakkoyun]

@pracucci @pstibrany I'm trying to come up with sane default for memcached, could point me to the right direction? What works for you best?

[pracucci]

It really depends on the scale you wanna address. I still think the defaults I've set into Thanos are OK for a small/medium scale:
https://github.com/thanos-io/thanos/blob/1a2edf19475b854181e5abaa4b6f1c9caf2a1547/pkg/cacheutil/memcached_client.go#L35-L44

If you wanna be a bit more aggressive you may consider:

timeout: '2s',
maxIdleConnections: 1000,
maxAsyncConcurrency: 100,
maxAsyncBufferSize: 100000,
maxGetMultiConcurrency: 900,
maxGetMultiBatchSize: 1000,

I've also seen you set dnsProviderUpdateInterval: '3s'. I'm not sure there's a real value of being that aggressive here. The default 10s could be enough (but maybe you find a good reason to lower it).

[kakkoyun]

Thanks a lot for the response, this was really helpful.

[kakkoyun]

This is in decent shape to merge, could you have another look @metalmatze?

[bwplotka]

Nice, some suggestions only.

[brancz]

nice!