Add AM aggregation group metrics

[emanlodovice]

What this PR does:
This PR emits the alertmanager_dispatcher_aggregation_groups and alertmanager_dispatcher_alert_processing_duration_seconds metrics from alertmanager dispatcher.

Which issue(s) this PR fixes:
Fixes #5588

Checklist

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

[emanlodovice]

Not sure what is wrong with the tests, ci / test sometimes passes and sometimes fails

[yeya24]

==================
WARNING: DATA RACE
Read at 0x00c000cf0238 by goroutine 40067:
  github.com/thanos-io/thanos/pkg/block/indexheader.(*parallelBucketReader).GetRange.func1()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/parallel_bucket.go:73 +0x324
  golang.org/x/sync/errgroup.(*Group).Go.func1()
      /__w/cortex/cortex/vendor/golang.org/x/sync/errgroup/errgroup.go:75 +0x82

Previous write at 0x00c000cf0238 by goroutine 40024:
  github.com/thanos-io/thanos/pkg/block/indexheader.(*parallelBucketReader).GetRange()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/parallel_bucket.go:80 +0x25c
  github.com/thanos-io/thanos/pkg/block/indexheader.(*chunkedIndexReader).CopySymbols()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:220 +0x1d4
  github.com/thanos-io/thanos/pkg/block/indexheader.WriteBinary()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:104 +0x404
  github.com/thanos-io/thanos/pkg/block/indexheader.NewBinaryReader()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:547 +0x506
  github.com/thanos-io/thanos/pkg/block/indexheader.(*ReaderPool).NewBinaryReader()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/reader_pool.go:89 +0x21a
  github.com/thanos-io/thanos/pkg/store.(*BucketStore).addBlock()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/store/bucket.go:744 +0x747
  github.com/thanos-io/thanos/pkg/store.(*BucketStore).SyncBlocks.func1()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/store/bucket.go:621 +0x7b

Goroutine 40067 (running) created at:
  golang.org/x/sync/errgroup.(*Group).Go()
      /__w/cortex/cortex/vendor/golang.org/x/sync/errgroup/errgroup.go:72 +0x12e
  github.com/thanos-io/thanos/pkg/block/indexheader.(*parallelBucketReader).GetRange()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/parallel_bucket.go:69 +0x236
  github.com/thanos-io/thanos/pkg/block/indexheader.(*chunkedIndexReader).CopySymbols()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:220 +0x1d4
  github.com/thanos-io/thanos/pkg/block/indexheader.WriteBinary()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:104 +0x404
  github.com/thanos-io/thanos/pkg/block/indexheader.NewBinaryReader()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/binary_reader.go:547 +0x506
  github.com/thanos-io/thanos/pkg/block/indexheader.(*ReaderPool).NewBinaryReader()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/reader_pool.go:89 +0x21a
  github.com/thanos-io/thanos/pkg/store.(*BucketStore).addBlock()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/store/bucket.go:744 +0x747
  github.com/thanos-io/thanos/pkg/store.(*BucketStore).SyncBlocks.func1()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/store/bucket.go:621 +0x7b

Goroutine 40024 (running) created at:
  github.com/thanos-io/thanos/pkg/store.(*BucketStore).SyncBlocks()
      /__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/store/bucket.go:619 +0x[126](https://github.com/cortexproject/cortex/actions/runs/6437075545/job/17481607608?pr=5592#step:6:127)
  github.com/cortexproject/cortex/pkg/storegateway.(*BucketStores).SyncBlocks.func1()
      /__w/cortex/cortex/pkg/storegateway/bucket_stores.go:171 +0x44
  github.com/cortexproject/cortex/pkg/storegateway.(*BucketStores).syncUsersBlocks.func2()
      /__w/cortex/cortex/pkg/storegateway/bucket_stores.go:242 +0x17c
==================
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xba8907]

goroutine 40067 [running]:
github.com/thanos-io/thanos/pkg/runutil.CloseWithErrCapture(0xc000295ee8, {0x0, 0x0}, {0x27be0e5, 0xc}, {0x0, 0x0, 0x0})
	/__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/runutil/runutil.go:[144](https://github.com/cortexproject/cortex/actions/runs/6437075545/job/17481607608?pr=5592#step:6:145) +0x307
github.com/thanos-io/thanos/pkg/block/indexheader.(*parallelBucketReader).GetRange.func1()
	/__w/cortex/cortex/vendor/github.com/thanos-io/thanos/pkg/block/indexheader/parallel_bucket.go:73 +0x3a6
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/__w/cortex/cortex/vendor/golang.org/x/sync/errgroup/errgroup.go:75 +0x83
created by golang.org/x/sync/errgroup.(*Group).Go
	/__w/cortex/cortex/vendor/golang.org/x/sync/errgroup/errgroup.go:72 +0x12f
FAIL	github.com/cortexproject/cortex/pkg/storegateway	213.665s

Seems related to the block parallel download feature. I will take a look

[yeya24]

LGTM