low-cost app api endpoint to determine probe connectedness

[rade]

In Weave Cloud we'd benefit from having an API endpoint that tells is whether there are any probes connected. ATM the UI is invoking /api/probes for that, which entails fetching and merging the reports for the current app window and hence is expensive.

[rade]

I wonder whether expanding the /api endpoint might be the way to go. That endpoint returns a bunch of readily available info. The exception is plugins, which requires report fetching; we could have a /api?sparse variant that excludes that.

[rade]

In order to determine "whether there are any probes connected", we could expand the app.Reporter interface with a HasReports(context.Context, time.Time) (bool, error) method - if we have reports we must have probes.

In case of of the awsCollector, the implementation of that method would simply execute the first portion of Report(), which queries Dynamo for the report keys, which is a fast operation.