Data source: Handle datasource withCredentials option properly

[hvtuananh]

The fetch() API won't send cookies or other type of credentials unless
you set the credentials init option. Some datasources like Prometheus
and Elasticsearch have withCredentials option in Browser access mode,
but this option is not currently getting passed in the fetch() API.

What this PR does / why we need it:

This PR corrects the behavior of the With Credentials checkbox on some datasources like Prometheus and Elasticsearch.

Which issue(s) this PR fixes:

Fixes #23338.

Special notes for your reviewer:

This is my first time contributing to the Grafana project, so comments and suggestions are appreciated.

[CLAassistant]

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All committers have signed the CLA.

[mckn]

@hvtuananh welcome to the project! Really great of you to jump in an helping out 🙌

What do you think about changing it from withCredentials to credentials and pass limit|omit|same-origin instead? Just to make this a bit more flexible?

[hvtuananh]

@mckn the options.withCredentials var is passed all the way from datasource plugins, like https://github.com/grafana/grafana/blob/master/public/app/plugins/datasource/prometheus/datasource.ts#L147. Do you suggest we change all references of withCredentials to credentials?

Also I found that during testing, the parseInitFromOptions is used for more than just datasources (like save new passwords), so I'm picturing we can do something like:

if withCredentials is:
    undef: set credentials to same-origin (default)
    false: set credentials to omit
    true:  set credentials to include

[mckn]

@mckn the options.withCredentials var is passed all the way from datasource plugins, like https://github.com/grafana/grafana/blob/master/public/app/plugins/datasource/prometheus/datasource.ts#L147. Do you suggest we change all references of withCredentials to credentials?

Also I found that during testing, the parseInitFromOptions is used for more than just datasources (like save new passwords), so I'm picturing we can do something like:

if withCredentials is:
    undef: set credentials to same-origin (default)
    false: set credentials to omit
    true:  set credentials to include

Aha, mybad 🙈 I thought it was a newly added option but now I see that it was used previously since angular http was supporting that flag. Then I totally agree with you that this is the best way of doing it.

[hvtuananh]

Thank you for reviewing!