add dashboard url in external labels

[neo-liang-sap]

How to categorize this PR?
/area monitoring

What this PR does / why we need it:
based on some discussion with @vicwicker i'm adding dashboard url into external labels field, might also be valuable for the open source users so that they can easily add links to the dashboard in their alert descriptions if they want to
Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

add dashboard url in external labels

[gardener-prow]

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[neo-liang-sap]

/retest-required

[neo-liang-sap]

@vicwicker do you have any idea why some test are failing? from error msg i just saw sth like shoot can't be reconciled but unable to find any relation with my pr code change

[shafeeqes]

/retest

[neo-liang-sap]

/retest

[vicwicker]

Oh I am sorry @neo-liang-sap but I had an internal discussion with @istvanballok about this and he made a point that adding the URL as external label would be adding it everywhere (i.e., also for all metrics) which is not desired. We only need it for the alerts so I started preparing a different change to achieve this using alert_relabel_reconfigs but I didn't open a PR yet.

Unfortunately, I don't think we are going to go this route. I am very sorry I forgot to mention this in our previous chat.

[neo-liang-sap]

@vicwicker no problem, thanks for the message! so i will close this PR, right?

[vicwicker]

@neo-liang-sap Yes, please. Thanks a lot!