Add changelog for alertmanager sharding and add experimental warning log. (#4222)

Signed-off-by: Steve Simpson <[email protected]>

Review comments.

Signed-off-by: Steve Simpson <[email protected]>

Fix doing numbers bad.

Signed-off-by: Steve Simpson <[email protected]>

Author: stevesg

CHANGELOG.md

@@ -13,6 +13,12 @@
 * [FEATURE] Alertmanager: Added `-alertmanager.max-config-size-bytes` limit to control size of configuration files that Cortex users can upload to Alertmanager via API. This limit is configurable per-tenant. #4201
 * [FEATURE] Alertmanager: Added `-alertmanager.max-templates-count` and `-alertmanager.max-template-size-bytes` options to control number and size of templates uploaded to Alertmanager via API. These limits are configurable per-tenant. #4223
 * [FEATURE] Added flag `-debug.block-profile-rate` to enable goroutine blocking events profiling. #4217
+* [FEATURE] Alertmanager: The experimental sharding feature is now considered complete. Detailed information about the configuration options can be found [here for alertmanager](https://cortexmetrics.io/docs/configuration/configuration-file/#alertmanager_config) and [here for the alertmanager storage](https://cortexmetrics.io/docs/configuration/configuration-file/#alertmanager_storage_config). To use the feature: #3925 #4020 #4021 #4031 #4084 #4110 #4126 #4127 #4141 #4146 #4161 #4162 #4222
+  * Ensure that a remote storage backend is configured for Alertmanager to store state using `-alertmanager-storage.backend`, and flags related to the backend. Note that the `local` and `configdb` storage backends are not supported.
+  * Ensure that a ring store is configured using `-alertmanager.sharding-ring.store`, and set the flags relevant to the chosen store type.
+  * Enable the feature using `-alertmanager.sharding-enabled`.
+  * Note the prior addition of a new configuration option `-alertmanager.persist-interval`. This sets the interval between persisting the current alertmanager state (notification log and silences) to object storage. See the [configuration file reference](https://cortexmetrics.io/docs/configuration/configuration-file/#alertmanager_config) for more information.
+* [ENHANCEMENT] Alertmanager: Cleanup persisted state objects from remote storage when a tenant configuration is deleted. #4167
 * [ENHANCEMENT] Storage: Added the ability to disable Open Census within GCS client (e.g `-gcs.enable-opencensus=false`). #4219
 * [ENHANCEMENT] Alertmanager: introduced new metrics to monitor operation when using `-alertmanager.sharding-enabled`: #4149
   * `cortex_alertmanager_state_fetch_replica_state_total`

pkg/alertmanager/multitenant.go

@@ -37,6 +37,7 @@ import (
 	"github.com/cortexproject/cortex/pkg/util"
 	"github.com/cortexproject/cortex/pkg/util/concurrency"
 	"github.com/cortexproject/cortex/pkg/util/flagext"
+	util_log "github.com/cortexproject/cortex/pkg/util/log"
 	"github.com/cortexproject/cortex/pkg/util/services"
 )
 
@@ -323,6 +324,8 @@ func NewMultitenantAlertmanager(cfg *MultitenantAlertmanagerConfig, store alerts
 
 	var ringStore kv.Client
 	if cfg.ShardingEnabled {
+		util_log.WarnExperimentalUse("Alertmanager sharding")
+
 		ringStore, err = kv.NewClient(
 			cfg.ShardingRing.KVStore,
 			ring.GetCodec(),