Alerting: Support values in notification templates

[grobinson-grafana]

What this PR does / why we need it:

We have received a lot of feedback regarding the ValueString in alert notifications. Perhaps one of the most frequent complaints about ValueString is that it is difficult to read because it contains a lot of information, and the information is shown as a JSON-like string. Users have often asked how it can be templated and the answer is that it can't.

Until now users have been able to add custom annotations to their alert rules which contains values via the $values variable added in previous versions of Grafana. However, these custom annotations must be added for each of the user's alert rule, instead of once in a template that all of their alerts can be notified via.

This commit adds then the much requested feature to support values in notification templates. Users can then create a single template that prints the annotations, labels and values of their alerts in a format of their choice!

Here is an example of the default template with an instant vector:

**Firing**

Value: A=1
Labels:
 - alertname = Test
 - grafana_folder = General Alerting
Annotations:
Source: http://localhost:3000/alerting/grafana/wXZdwwVVz/view
Silence: http://localhost:3000/alerting/silence/new?alertmanager=grafana&matcher=alertname%3DTest&matcher=grafana_folder%3DGeneral+Alerting

Here is an example of the default template with a reduce expression:

**Firing**

Value: B=1
Labels:
 - alertname = Test
 - grafana_folder = General Alerting
Annotations:
Source: http://localhost:3000/alerting/grafana/wXZdwwVVz/view
Silence: http://localhost:3000/alerting/silence/new?alertmanager=grafana&matcher=alertname%3DTest&matcher=grafana_folder%3DGeneral+Alerting

Here is an example of the default template with a reduce and math expression:

**Firing**

Value: B=1, C=1
Labels:
 - alertname = Test
 - grafana_folder = General Alerting
Annotations:
Source: http://localhost:3000/alerting/grafana/wXZdwwVVz/view
Silence: http://localhost:3000/alerting/silence/new?alertmanager=grafana&matcher=alertname%3DTest&matcher=grafana_folder%3DGeneral+Alerting

Last is an example of the default template with a non-reduced time series:

**Firing**

Value: [no value]
Labels:
 - alertname = DatasourceError
 - grafana_folder = General Alerting
 - rulename = Test
Annotations:
Source: http://localhost:3000/alerting/grafana/wXZdwwVVz/view
Silence: http://localhost:3000/alerting/silence/new?alertmanager=grafana&matcher=alertname%3DDatasourceError&matcher=grafana_folder%3DGeneral+Alerting&matcher=rulename%3DTest

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/36490

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/36595

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/36599

[gillesdemey]

LGTM!

[yuri-tceretian]

Can you please update the docs in UI

here

grafana/public/app/features/alerting/unified/components/receivers/TemplateData.ts

Lines 114 to 118 in 4c4b758

	{
	name: 'ValueString',
	type: 'string',
	notes: 'String that contains the labels and value of each reduced expression in the alert.',
	},

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/36913

[jtheory]

Awesome, thank you; this'll be hugely helpful 🏆

It looks to me like this doesn't risk breaking any existing alerting rule / templates -- the only difference it would make is that anyone who's currently dumping the entire ValueString into a notification will get a differently formatted result, right?

The main question I have is about updating docs & providing examples there, plus (ideally) how we should update the UI to help users 1) learn this is now possible and 2) help them do it correctly
(UI updates aren't a blocker for this PR; I'm not sure if basic doc updates are a blocker or not)

[grobinson-grafana]

It looks to me like this doesn't risk breaking any existing alerting rule / templates -- the only difference it would make is that anyone who's currently dumping the entire ValueString into a notification will get a differently formatted result, right?

All users who are using ValueString in their custom templates will see no change as I suspect we are neither changing nor removing ValueString until at least Grafana 10.

However, all users who are using the default template, or even just default.message in their custom templates, will see this change. Any users who were using ValueString can update their templates to use __text_values_list and also see the change.

I will work with @brendamuir to also document this in the Whats New page. I would like to get it into 9.2.0 if there will be a beta2, otherwise I think we should wait for 9.3.0.