Reduced memory allocations by user.ExtractFromGRPCRequest()

[pracucci]

What this PR does:

In Mimir ingesters, 10% of object (and memory) allocations come from gRPC metadata.FromIncomingContext(). metadata.FromIncomingContext() is called in few places, including user.ExtractFromGRPCRequest(). The problem of metadata.FromIncomingContext() is that it creates a copy of all metadata map, when we only want to lookup a single key.

Two years ago, metadata.ValueFromIncomingContext() was introduced in gRPC golang library exactly for this reason. The function is still marked as experimental but it was left untouched since was introduced 2 years ago. I propose to use it. If it will get removed, we can always revert back to metadata.FromIncomingContext().

Benchmark:

goos: darwin
goarch: amd64
pkg: github.com/grafana/dskit/user
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
                          │ before.txt  │             after.txt              │
                          │   sec/op    │   sec/op     vs base               │
ExtractFromGRPCRequest-12   772.2n ± 2%   124.0n ± 7%  -83.94% (p=0.002 n=6)

                          │ before.txt  │             after.txt             │
                          │    B/op     │    B/op     vs base               │
ExtractFromGRPCRequest-12   560.00 ± 0%   80.00 ± 0%  -85.71% (p=0.002 n=6)

                          │ before.txt  │             after.txt             │
                          │  allocs/op  │ allocs/op   vs base               │
ExtractFromGRPCRequest-12   10.000 ± 0%   3.000 ± 0%  -70.00% (p=0.002 n=6)

Which issue(s) this PR fixes:

N/A

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]