Alertmanager 0.20.0

[Simon Pasquier]

Hello,

We are pleased to announce the release of Alertmanager v0.20.0. It's
available at:

https://github.com/prometheus/alertmanager/releases/tag/v0.20.0

Changelog:
* [CHANGE] Check that at least one silence matcher matches a non-empty
string. #2081
* [ENHANCEMENT] [pagerduty] Check that PagerDuty keys aren't empty. #2085
* [ENHANCEMENT] [template] Add the `stringSlice` function. #2101
* [ENHANCEMENT] Add `alertmanager_dispatcher_aggregation_groups` and
`alertmanager_dispatcher_alert_processing_duration_seconds` metrics.
#2113
* [ENHANCEMENT] Log unused receivers. #2114
* [ENHANCEMENT] Add `alertmanager_receivers` metric. #2114
* [ENHANCEMENT] Add `alertmanager_integrations` metric. #2117
* [ENHANCEMENT] [email] Add Message-Id Header to outgoing emails. #2057
* [BUGFIX] Don't garbage-collect alerts from the store. #2040
* [BUGFIX] [ui] Disable the grammarly plugin on all textareas. #2061
* [BUGFIX] [config] Forbid nil regexp matchers. #2083
* [BUGFIX] [ui] Fix Silences UI when several filters are applied. #2075

Thanks to all the contributors.

Simon