Support HAR file generation as a BiDi command

[juliandescottes]

The topic of HAR support often comes up when we discuss about network event support (#196). I am filing this issue to summarize mozilla's opinion and intent on that topic.

At the moment, we (mozilla) are prototyping HAR generation based on the network events proposed in #196 to make sure that clients can generate HAR files using BiDi. But we don't plan to directly add HAR support in BiDi, we just want to ensure network events are compatible with it.

But if there is a strong interest for generating HAR with BiDi however, it might make sense add a command in the future. Some thoughts about pros of having a command:

• HAR format is specified, so if we don't have a command any client (which wants to provide the HAR feature) will roughly have to reimplement the same logic (which will therefore be duplicated across clients)
• related to the previous point, if we want to promote HAR usage, forcing clients to reimplement that logic can be an issue for adoption
• it will be hard to test against regression for HAR support if we don't own such a client
• generating a HAR in the browser might be more efficient because you don't have to communicate all the network events to the client

Now some cons:

• if we have a HAR command, then all vendors will need to implement the HAR generation logic
• HAR files can be huge when recording complex scenarios (several 100MBs)

As a summary there is no intention from the mozilla side to have a BiDi HAR command "for now". We only want to ensure network events from BiDi can be used for HAR generation. We can reconsider after discussing this as a group, based on the interest.