Format for time series data with non-trivial sampled data
Based on this article about Data Formats in Data Science, the short answer to your question is: use JSON.

This may seem natural next step after csv and no more, but it is actually quite interesting:

JSON is widely supported in many programming languages, it is a valid MIME type in Internet standards (application/json, data stream mentioned in your question) and it has several standard and non standard extensions (geoJSON, binary json). If there is not a standard structure, JSON makes it easy to invent your own, because it is easily modifiable.

According to the article JSON works as value in HADOOP. For HADOOP it is suitable, because basically JSON is plain text when you think it as a data item.

Also article mentions JSON more efficient than CSV.