ETA for progress bars

[tkf]

It would be nice to print ETA like ProgressMeter.jl does. To do this, I think we need to record progress value and time() when the logger received

struct ProgressBar
    init_progress::Float64
    init_time::Float64
end

struct TerminalLogger <: AbstractLogger
    ...
    bars::Dict{Symbol, ProgressBar}
end

Current TerminalLogger is nice as it is mostly "stateless" (except sticky_messages) and bars would break this property. But I think it would be required anyway at some point to support something like nested progress bars JuliaLogging/ProgressLogging.jl#7 (comment) / JuliaLogging/ProgressLogging.jl#8.

[c42f]

Yes, an ETA would be cool.

I'm fine with TerminalLogger being more stateful, I don't think we can make a good UI without tracking a fair bit of state.

For access to sticky_messages, we perhaps should move that out of TerminalLogger anyway and make it global in some way. It's a model of the connected terminal and there's generally only one of those (I guess there could be more but that seems like a funky thing we could deal with later).

Yikes, dynamic gensym() observed in ProgressLogging #7 is not ideal, let's fix that :-)