Allow loading traces from a file

[nthuemmel-scontain]

Currently, Venator needs to be started and attached to from a running process.

It would be great to be able to log everything to a file from an application, and then later open the file in Venator for a post-mortem analysis.
One idea would be to use the JSON format from tracing_subscriber (though that probably does not capture span enter/exit timing information)

[kmdreko]

I have gotten other requests for loading from a JSON-lines encoded file, so I'll probably get to it at some point. Though it will likely be built around Bunyan (or some other standardized format) since the tracing-subscriber format is not really useful for someone not using Rust.

[RustyNoedar]

@kmdreko Hi, thanks for your work on this awesome project. I was wondering, is there any progress on this issue?