More control over Druid metadata refresh features

[Mogball]

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• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
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Superset version

0.19.3

Periodic metadata refreshing was mentioned in #619.

There should also be a feature to only check for new Druid datasources, instead of only being able to force refresh everything. A small refresh button next to each datasource and cluster would also be nice.

[mistercrunch]

Notice: this issue has been closed because it has been inactive for 212 days. Feel free to comment and request for this issue to be reopened.