Release Notes for v2.8 (#4740) (#4901)

* Add release note md for 2.8

Signed-off-by: Jon Kartago Lamida <me@lamida.net>

* Fix the title

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* Add some note in the release note

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* Update docs/sources/mimir/release-notes/v2.8.md

Co-authored-by: Peter Štibraný <pstibrany@gmail.com>

* Apply PR feedback

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* Small adjustment

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* More small adjustment

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* Add config removal in release note

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* Update docs/sources/mimir/release-notes/v2.8.md

Co-authored-by: Charles Korn <charleskorn@users.noreply.github.com>

* Update docs/sources/mimir/release-notes/v2.8.md

Co-authored-by: Charles Korn <charleskorn@users.noreply.github.com>

* Move out non feature/enhancement changes

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* Remove native histogram from bugs and add protobuf to enhancement section

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* Add msi-resource into release note deprecation note

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* Update docs/sources/mimir/release-notes/v2.8.md

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* Update docs/sources/mimir/release-notes/v2.8.md

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* Update docs/sources/mimir/release-notes/v2.8.md

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* Run make doc

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* Apply recent PR feedbacks

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* Move experimental to the top of release note

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Signed-off-by: Jon Kartago Lamida <me@lamida.net>
Co-authored-by: Peter Štibraný <pstibrany@gmail.com>
Co-authored-by: Charles Korn <charleskorn@users.noreply.github.com>
Co-authored-by: George Krajcsovits <krajorama@users.noreply.github.com>
(cherry picked from commit ae9978b)

Co-authored-by: Jon Kartago Lamida <jon.lamida@grafana.com>

docs/sources/mimir/release-notes/v2.8.md

@@ -0,0 +1,58 @@
+---
+title: "Grafana Mimir version 2.8 release notes"
+menuTitle: "V2.8 release notes"
+description: "Release notes for Grafana Mimir version 2.8"
+weight: 300
+---
+
+# Grafana Mimir version 2.8 release notes
+
+Grafana Labs is excited to announce version 2.8 of Grafana Mimir.
+
+The highlights that follow include the top features, enhancements, and bugfixes in this release. For the complete list of changes, see the [changelog](https://github.com/grafana/mimir/blob/main/CHANGELOG.md).
+
+## Features and enhancements
+
+- **Experimental support for using Redis as cache** Mimir now can use Redis for caching results, chunks, index and metadata.
+- **Experimental support for fetching secret from Vault for TLS configuration**.
+- **Experimental support for querying native histograms**. This support is not finalized as the related Prometheus API is also experimental, thus the exact behavior might change in future releases.
+- **Query-frontend and ruler now use protobuf internal query result payload format by default** This reduces the CPU and memory utilisation of the querier, query-frontend and ruler, as well as reducing network bandwidth consumed between these components.
+- **Query-frontend cached results now contain timestamp** This allows Mimir to check if cached results are still valid based on current TTL configured for tenant. Results cached by previous Mimir version are used until they expire from cache, which can take up to 7 days. If you need to use per-tenant TTL sooner, please flush results cache manually.
+- **Optimized regular expression label matchers** This reduces CPU utilisation in ingesters and store-gateways when running queries containing regular expression label matchers.
+- **Store-gateway now use streaming for LabelNames RPC** This improves memory utilization in store-gateway when calling LabelNames RPC.
+
+### Helm chart improvements
+
+The Grafana Mimir and Grafana Enterprise Metrics Helm chart is now released independently. See the [Grafana Mimir Helm chart documentation](/docs/helm-charts/mimir-distributed/latest/).
+
+## Important changes
+
+In Grafana Mimir 2.8 we have removed the following previously deprecated or experimental metrics:
+
+- `cortex_bucket_store_series_get_all_duration_seconds`
+- `cortex_bucket_store_series_merge_duration_seconds`
+- `cortex_ingester_tsdb_wal_replay_duration_seconds`
+
+The following configuration options are deprecated and will be removed in Grafana Mimir 2.10:
+
+- The CLI flag `-blocks-storage.tsdb.max-tsdb-opening-concurrency-on-startup` and its respective YAML configuration option `tsdb.max_tsdb_opening_concurrency_on_startup`.
+
+The following configuration options that were deprecated in 2.6 are removed:
+
+- The CLI flag `-store.max-query-length` and its respective YAML configuration option `limits.max_query_length`.
+
+The following configuration options that were deprecated in 2.5 are removed:
+
+- The CLI flag `-azure.msi-resource`.
+
+The following experimental options and features are now stable:
+
+- The protobuf internal query result payload format, which is now enabled by default
+
+We changed default value of block storage retention period. The default value for `-blocks-storage.tsdb.retention-period` was `24h` and now is `13h`
+
+## Bug fixes
+
+- Querier: Streaming remote read will now continue to return multiple chunks per frame after the first frame. [PR 4423](https://github.com/grafana/mimir/pull/4423)
+- Query-frontend: don't retry queries which error inside PromQL. [PR 4643](https://github.com/grafana/mimir/pull/4643)
+- Store-gateway & query-frontend: report more consistent statistics for fetched index bytes. [PR 4671](https://github.com/grafana/mimir/pull/4671)