Field caps request returns too much data

[lightindigo]

After changes introduced in #20, when w're trying to request logs from otel indices for a large period eg 30 days, request for _field_caps either takes a lot of time, holding logs request or even fails due to its response size, because attributes.* field may return infinite amount of fields due to it's highly dynamic nature. I believe it would be nice to have ability to either disable the autocomplete feature for specific datasource or have ability to pick fields/index source for autocomplete to make better experience here.