Uploader: Add `total_blob_bytes` field to `Experiment` proto (#3448)
Uploader: Add `total_blob_bytes` field to `Experiment` proto (#3448)

* Motivation for features / changes
  * Towards fixing b/152749189: displaying the number of blob bytes that an experiment contains int the `tensorboard dev list` command output
* Technical description of changes
  * Add a `total_blob_bytes` field to the `Experiment` proto, which is the respnose message type of `StreamExperiemnts`
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard