Highlights:

Rare overlapping block issue fixed
Deduplication bug with counters fixed
Receiver is now multi-tenant (multi TSDB)
Added @memcached meta and chunk caching. (e.g allows to significantly limit objstore traffic)
TSDB isolation added for Ruler and Receive
Querier performance significantly improved especially when used with duplicated Store APIs (e.g for HA purposes).

Thanks all for contributing! 🤗

Fixed

#2548 Query: Fixed rare cases of double counter reset accounting when querying rate with deduplication enabled.
#2536 S3: Fixed AWS STS endpoint url to https for Web Identity providers on AWS EKS.
#2501 Query: Gracefully handle additional fields in SeriesResponse protobuf message that may be added in the future.
#2568 Query: Don't close the connection of strict, static nodes if establishing a connection had succeeded but Info() call failed.
#2615 Rule: Fix bugs where rules were out of sync.
#2614 Tracing: Disabled Elastic APM Go Agent default tracer on initialization to disable the default metric gatherer.
#2525 Query: Fixed logging for dns resolution error in the Query component.
#2484 Query/Ruler: Fixed issue #2483, when web.route-prefix is set, it is added twice in HTTP router prefix.
#2416 Bucket: Fixed issue #2416 bug in inspect --sort-by doesn't work correctly in all cases.
#2719 Query: irate and resets use now counter downsampling aggregations.
#2705 minio-go: Added support for af-south-1 and eu-south-1 regions.
#2753 Sidecar, Receive, Rule: Fixed possibility of out of order uploads in error cases. This could potentially cause Compactor to create overlapping blocks.

#2012 Receive: Added multi-tenancy support (based on header)
#2502 StoreAPI: Added hints field to SeriesResponse. Hints in an opaque data structure that can be used to carry additional information from the store and its content is implementation specific.
#2521 Sidecar: Added thanos_sidecar_reloader_reloads_failed_total, thanos_sidecar_reloader_reloads_total, thanos_sidecar_reloader_watch_errors_total, thanos_sidecar_reloader_watch_events_total and thanos_sidecar_reloader_watches metrics.
#2412 UI: Added React UI from Prometheus upstream. Currently only accessible from Query component as only /graph endpoint is migrated.
#2532 Store: Added hidden option --store.caching-bucket.config=<yaml content> (or --store.caching-bucket.config-file=<file.yaml>) for experimental caching bucket, that can cache chunks into shared memcached. This can speed up querying and reduce number of requests to object storage.
#2579 Store: Experimental caching bucket can now cache metadata as well. Config has changed from #2532.
#2526 Compact: In case there are no labels left after deduplication via --deduplication.replica-label, assign first replica-label with value deduped.
#2621 Receive: Added flag to configure forward request timeout. Receive write will complete request as soon as a quorum of writes succeeds.

#2194 Updated to golang v1.14.2.
#2505 Store: Removed obsolete thanos_store_node_info metric.
2513 Tools: Moved thanos bucket commands to thanos tools bucket, also
moved thanos check rules to thanos tools rules-check. thanos tools rules-check also takes rules by --rules repeated flag not argument
anymore.
#2548 Store, Querier: remove duplicated chunks on StoreAPI.
#2596 Updated Prometheus dependency to @cd73b3d33e064bbd846fc7a26dc8c313d46af382 which falls in between v2.17.0 and v2.18.0.
- Receive,Rule: TSDB now supports isolation of append and queries.
- Receive,Rule: TSDB now holds less WAL files after Head Truncation.
#2450 Store: Added Regex-set optimization for label=~"a|b|c" matchers.
#2526 Compact: In case there are no labels left after deduplication via --deduplication.replica-label, assign first replica-label with value deduped.
2603 Store/Querier: Significantly optimize cases where StoreAPIs or blocks returns exact overlapping chunks (e.g Store GW and sidecar or brute force Store Gateway HA).