Memcached client error message should include the cache name

[bboreham]

Describe the bug
This error message doesn't say which cache was involved (index, metadata, chunks, etc.):

level=warn ts=2021-05-12T14:47:45.076049765Z caller=memcached_client.go:382 msg="failed to fetch items from memcached" numKeys=2535 firstKey=S:01F5FBARXPESQTKV2EH44JB0B3:28400064 err="read tcp 10.244.221.99:60966->10.244.222.6:11211: i/o timeout"

This was with Cortex v1.8.0.

Expected behavior
Add a tag like cache=index.

Storage Engine

• Blocks
• Chunks

[pracucci]

This should have been fixed by thanos-io/thanos#3929 which has been vendored in #4102.

Closing for now, but let me know if it doesn't fix it.