Alertmanager - v0.26.0-rc.0

[CHANGE] Telegram Integration: api_url is now optional. #2981
[CHANGE] Telegram Integration: ParseMode default is now HTML instead of MarkdownV2. #2981
[CHANGE] Webhook Integration: url is now marked as a secret. It will no longer show up in the logs as clear-text. #3228
[CHANGE] Metrics: New label reason for alertmanager_notifications_failed_total metric to indicate the type of error of the alert delivery. #3094 #3307
[FEATURE] Clustering: New flag --cluster.label, to help to block any traffic that is not meant for the cluster. #3354
[FEATURE] Integrations: Add Microsoft Teams as a supported integration. #3324
[ENHANCEMENT] Telegram Integration: Support bot_token_file for loading this secret from a file. #3226
[ENHANCEMENT] Webhook Integration: Support url_file for loading this secret from a file. #3223
[ENHANCEMENT] Webhook Integration: Leading and trailing white space is now removed for the contents of url_file. #3363
[ENHANCEMENT] Pushover Integration: Support options device and sound (sound was previously supported but undocumented). #3318
[ENHANCEMENT] Pushover Integration: Support user_key_file and token_file for loading this secret from a file. #3200
[ENHANCEMENT] Slack Integration: Support errors wrapped in successful (HTTP status code 200) responses. #3121
[ENHANCEMENT] API: Add CORS and Cache-Control HTTP headers to all version 2 API routes. #3195
[ENHANCEMENT] UI: Receiver name is now visible as part of the alerts page. #3289
[ENHANCEMENT] Templating: Better default text when using {{ .Annotations }} and {{ .Labels }}. #3256
[ENHANCEMENT] Templating: Introduced a new function trimSpace which removes leading and trailing white spaces. #3223
[ENHANCEMENT] CLI: amtool silence query now supports the --id flag to query an individual silence. #3241
[ENHANCEMENT] Metrics: Introduced alertmanager_nflog_maintenance_total and alertmanager_nflog_maintenance_errors_total to monitor maintenance of the notification log. #3286
[ENHANCEMENT] Metrics: Introduced alertmanager_silences_maintenance_total and alertmanager_silences_maintenance_errors_total to monitor maintenance of silences. #3285
[ENHANCEMENT] Logging: Log GroupKey and alerts on alert delivery when using debug mode. #3438
[BUGFIX] Configuration: Empty list of receivers and inhibit_rules would cause the alertmanager to crash. #3209
[BUGFIX] Templating: Fixed a race condition when using the title function. It is now race-safe. #3278
[BUGFIX] API: Fixed duplicate receiver names in the api/v2/receivers API endpoint. #3338
[BUGFIX] API: Attempting to delete a silence now returns the correct status code, 404 instead of 500. #3352
[BUGFIX] Clustering: Fixes a panic when tls_client_config is empty. #3443

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