Support OpenTSDB pre aggregated values

[seanlook]

Now aggregator plugin basicstats is more like downsampling across time for a single timeseries. But sometimes we need to pre-aggregate to group by some of tags within a specific time or a range time.

refer: http://opentsdb.net/docs/build/html/api_http/rollup.html

[glinton]

Is your use case to dump the basicstats output to TSDB's /api/rollup, or are you saying you'd like to see the "rollup" behavior mimicked?

[seanlook]

I mean data is rolled up in telegraf, and sent using normal /api/put. (OpenTSDB is not good at it, see rolled-up-pre-aggregates). Besides, many plugins do not use /api/rollup to save data

The ① is what basicstas does. The ② is what I mean:

[danielnelson]

If I'm understanding correctly, we need to keep more metadata about the created aggregates internally, somewhat similar to what was discussed in #3670, and then the opentsdb output could use the /api/rollup endpoint for sending the aggregate data.

[seanlook]

No need to keep much data in telegraf internally. The dimension to aggregate is fields, not time.

/api/rollup endpoint do that aggregation in opentsdb inside, but for now, telegraf do not use this endpoint?

[danielnelson]

Currently the opentsdb output uses only the /api/put endpoint.

[glinton]

@seanlook, does #4347 provide what you need?