Make field optional since if aggregation is doc_count it's should not…

[wasfree]

… be set

Fix for #1085 and #1035

If aggregation in metric_custom_indicator is set to doc_count it's not allowed to specifie "field"

[cla-checker-service]

💚 CLA has been signed

[tobio]

There's a few more changes required here, one of which is potentially large:

• Update where we populate the API model to optionally read the field (here and here). We'd likely also want to add some validation in so the provider errors if field is set when for doc_type aggregations, or unset for other aggregation types.
• Update the generated SLO client, the spec used to generate this client lists field as required, and so it's always being serialised in the API request body leading to 400's. That means updating slo-spec.yml with the current spec, regenerating the client with make generate-slo-client and fixing any errors caused by the potential model changes. Done in Update generated SLO client.  #1303
• Adding an acceptance test to slo_test.go verifying the actual bug fix.

[tobio]

@wasfree I've updated the SLO client as part of some other work, which should simplify the rest of this task if you've still got some time to look at it.

[kpatticha]

@tobio I’m also running into this limitation, are there any alternative workarounds until it gets merged?

[tobio]

Nope there's no workaround available unfortunately.