add templating for title and message fields

[wunter8]

Here's an initial working implementation, based on #171.

Right now, only the message and title fields can be templated, each using their own header (since this seemed like the quickest way to get something working).

In a future iteration, I think allowing other fields to be templated (e.g., priority, actions, attachment url, topic?, etc.) would be cool.

In that version, I was imagining an X-Template header that includes probably a JSON string for the different fields of a message (e.g., {"message":"${field.value} is ${field2}", "title":"An error occurred on ${hostname}", "priority":${error.level}, "attachment":"${error.screenshot.url}"})

[wunter8]

I updated the code to use the existing message and title fields.

Right now, if the message body is not valid JSON, the message and title fields will just contain the original template placeholders without anything being replaced and without any errors thrown. Let me know if you think this should change. (See line 2671 of server/server_test.go for an example (TestServer_MessageTemplate_MalformedJSONBody))

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 64.93%. Comparing base (4c0ec3f) to head (83356f5).

Files	Patch %	Lines
server/server.go	90.90%	3 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1057      +/-   ##
==========================================
+ Coverage   64.86%   64.93%   +0.06%     
==========================================
  Files          60       60              
  Lines        7944     7965      +21     
==========================================
+ Hits         5153     5172      +19     
- Misses       1930     1931       +1     
- Partials      861      862       +1     

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[binwiederhier]

This looks fantastic. Very simple and elegant.

The docs should include lots of real world examples; I can do that when I merge it in. Great work!

[binwiederhier]

What happens if result is a number or an array?

[wunter8]

This works fine. See TestServer_MessageTemplate_FancyGJSON (server/server_test.go:L2742)

[binwiederhier]

You should link https://gjson.dev/ in the docs. It's awesome.

Here's a real Grafana alert that I get for ntfy sometimes:

{"receiver":"ntfy\\.example\\.com/alerts","status":"resolved","alerts":[{"status":"resolved","labels":{"alertname":"Load avg 15m too high","grafana_folder":"Node alerts","instance":"10.108.0.2:9100","job":"node-exporter"},"annotations":{"summary":"15m load average too high"},"startsAt":"2024-03-15T02:28:00Z","endsAt":"2024-03-15T02:42:00Z","generatorURL":"localhost:3000/alerting/grafana/NW9oDw-4z/view","fingerprint":"becbfb94bd81ef48","silenceURL":"localhost:3000/alerting/silence/new?alertmanager=grafana\u0026matcher=alertname%3DLoad+avg+15m+too+high\u0026matcher=grafana_folder%3DNode+alerts\u0026matcher=instance%3D10.108.0.2%3A9100\u0026matcher=job%3Dnode-exporter","dashboardURL":"","panelURL":"","values":{"B":18.98211314475876,"C":0},"valueString":"[ var='B' labels={__name__=node_load15, instance=10.108.0.2:9100, job=node-exporter} value=18.98211314475876 ], [ var='C' labels={__name__=node_load15, instance=10.108.0.2:9100, job=node-exporter} value=0 ]"}],"groupLabels":{"alertname":"Load avg 15m too high","grafana_folder":"Node alerts"},"commonLabels":{"alertname":"Load avg 15m too high","grafana_folder":"Node alerts","instance":"10.108.0.2:9100","job":"node-exporter"},"commonAnnotations":{"summary":"15m load average too high"},"externalURL":"localhost:3000/","version":"1","groupKey":"{}:{alertname=\"Load avg 15m too high\", grafana_folder=\"Node alerts\"}","truncatedAlerts":0,"orgId":1,"title":"[RESOLVED] Load avg 15m too high Node alerts (10.108.0.2:9100 node-exporter)","state":"ok","message":"**Resolved**\n\nValue: B=18.98211314475876, C=0\nLabels:\n - alertname = Load avg 15m too high\n - grafana_folder = Node alerts\n - instance = 10.108.0.2:9100\n - job = node-exporter\nAnnotations:\n - summary = 15m load average too high\nSource: localhost:3000/alerting/grafana/NW9oDw-4z/view\nSilence: localhost:3000/alerting/silence/new?alertmanager=grafana\u0026matcher=alertname%3DLoad+avg+15m+too+high\u0026matcher=grafana_folder%3DNode+alerts\u0026matcher=instance%3D10.108.0.2%3A9100\u0026matcher=job%3Dnode-exporter\n"}

Untested, but I think:

https://ntfy.sh/mytopic?template=yes&title=Grafana+alert:+%24%28title%29&message=%24%28message%29

Would lead to this:

Title:

[RESOLVED] Load avg 15m too high Node alerts (10.108.0.2:9100 node-exporter)

Message:

**Resolved**

Value: B=18.98211314475876, C=0
Labels:
 - alertname = Load avg 15m too high
 - grafana_folder = Node alerts
 - instance = 10.108.0.2:9100
 - job = node-exporter
Annotations:
 - summary = 15m load average too high
Source: localhost:3000/alerting/grafana/NW9oDw-4z/view
Silence: localhost:3000/alerting/silence/new?alertmanager=grafana\u0026matcher=alertname%3DLoad+avg+15m+too+high\u0026matcher=grafana_folder%3DNode+alerts\u0026matcher=instance%3D10.108.0.2%3A9100\u0026matcher=job%3Dnode-exporter

[binwiederhier]

So elegant and simple. Well done. I'll merge it tomorrow

[wunter8]

Please have a look at the latest commit (dealing with message sizes). All of the tests still pass, and I added specific message size tests, so I don't think I broke anything. But another set of eyes is always good.

I think adding a custom GJSON modifier to the code would allow us to match values of the HTTP body and convert it to a priority level. I think including that in a separate PR might be good, though. We can release this and people can start using it before we add anything more.

[wunter8]

Oops. Forgot to take this out