Improve fluent mapping

[adamconnelly]

I've added a new way of configuring deserializers to try to simplify things a bit, and to make it easier for us to extend deserialization. So instead of writing the following:

MapOptional(metadata => metadata.Cloud, AzureEnvironment.AzureGlobalCloud, DetermineAzureCloud);

We can write it:

Map(metadata => metadata.Cloud)
    .WithDefault(AzureEnvironment.AzureGlobalCloud)
    .MapUsing(DetermineAzureCloud);

Originally when I'd added the MapOptional() / MapRequired() methods, I wanted to avoid going too far down the route of designing a fluent interface because this is just used internally by Promitor, but the approach of using overloads becomes pretty unwieldy very quickly. Making this change makes it easier to add extra functionality to the deserializers, for example custom validation. This would allow us to do something like this to make sure Cron expressions were in the correct format:

Map(scraping => scraping.Schedule)
    .IsRequired()
    .ValidateUsing(schedule => CronExpression.Parse(schedule), "The schedule must be a valid Cron expression.");

At the moment I've just converted the AzureMetadataDeserializer to give an example of how it might look, but I'll go ahead and convert all the deserializers if it's all looking good.

[CLAassistant]

All committers have signed the CLA.

[adamconnelly]

@tomkerkhove I haven't done end to end testing of this change yet. I'll do that before merging. I have made sure all the unit tests pass though, so I'm fairly confident the changes work fine since we've got pretty much complete coverage of deserialization.

Also, I realise I've accidentally committed this with the wrong gitconfig. I'll rebase and sort that to fix the CLA issue.

[promitor-bot]

Docker image for this PR was built and is available on Docker Hub.

You can pull it locally via the CLI:

docker pull tomkerkhove/promitor-agent-scraper-ci:pr1089-linux

Want to verify the new version? Run it locally:

docker run -d -p 8999:80 --name promitor-agent-scraper-pr1089-linux \
                         --env PROMITOR_AUTH_APPID='<azure-ad-app-id>' \
                         --env-file C:/Promitor/az-mon-auth.creds \
                         --volume C:/Promitor/metrics-declaration.yaml:/config/metrics-declaration.yaml  \
                         --volume C:/Promitor/runtime-config.yaml:/config/runtime.yaml \
                         tomkerkhove/promitor-agent-scraper-ci:pr1089-linux

You can find a CI version of our Helm chart on hub.helm.sh

[promitor-bot]

Docker image for this PR was built and is available on Docker Hub.

You can pull it locally via the CLI:

docker pull tomkerkhove/promitor-agent-scraper-ci:pr1089-linux

Want to verify the new version? Run it locally:

docker run -d -p 8999:80 --name promitor-agent-scraper-pr1089-linux \
                         --env PROMITOR_AUTH_APPID='<azure-ad-app-id>' \
                         --env-file C:/Promitor/az-mon-auth.creds \
                         --volume C:/Promitor/metrics-declaration.yaml:/config/metrics-declaration.yaml  \
                         --volume C:/Promitor/runtime-config.yaml:/config/runtime.yaml \
                         tomkerkhove/promitor-agent-scraper-ci:pr1089-linux

You can find a CI version of our Helm chart on hub.helm.sh

[tomkerkhove]

LGTM - Great addition Adam, thanks!

Let me know when I should merge.

[adamconnelly]

@tomkerkhove that's me tested it and it's looking good. Do you want me to create an issue for this work? I'm happy to do that.

[tomkerkhove]

No it's ok, thanks!

Created #1091 for the follow-up to align the other deserizalizers - Thanks again!