Can not add `Source.Results` to alerts

[saneksem]

I can not find a way to use notification properties like Source.Results listed in docs for Seq alerts triggered by query conditions.

I investigated source code and that's what I tried:

• Message template in app instance settings is attached as raw string value without any additional parsing. Even [RenderedMessage] is not being replaced.

AlertV2MessageBuilder
...
if (_messageTemplate != null)
{
    message.Attachments.Add(new SlackMessageAttachment(color, _messageTemplate, null, true));
}

• Listing including properties in app instance settings works as a filter for exceptions properties. If it's not populated all properties are included by default (except for SpecialProperties which I need).
• All attached properties in notification settings are actually filtered to NOT include special properties listed in seq docs.

foreach (var property in evt.Data.Properties)
            {
                if (SpecialProperties.Contains(property.Key)) continue;

Only Source.ContributingEvents are being processed in a separate block of code if they are included in event properties. Everything else is ignored because of SpecialProperties.
Using [Source.Results] or [@Source.Results] in text does nothing and I couldn't find any parsing for attached properties in code.

• Using different values/properties for message template in alert settings did not affect alert body at all.

I need to be able to include Source.Results like any alert property in HTML application template.
Reason: alert can have hundreds of contributing events every time it triggers but results may be very informative with just a few records using query with grouping and important fields selection.
Maybe we can add new checkbox "Include results" in notification/instance settings and process it similar to contributing events in AlertV2MessageBuilder.

[nblumhardt]

Since Slack messages are text-based, another option would be to port to the Seq.Syntax library and use the same approach as the Seq.App.Mail.* and Seq.App.HttpRequest apps, described in #68, e.g.:

https://github.com/datalust/seq-app-mail/blob/dev/src/Seq.Mail/Resources/DefaultTextBodyTemplate.txt

I'd jump in and have a shot at this, but I don't know how to achieve it without awkward breaking changes for existing usage. In the end, it may be better to stand up a parallel Slack integration app using the alternative template syntax 🤔

[nblumhardt]

This is done now in #71.