Improve tracing_subscriber wasm support by swapping `web_time` with `std::time`

[cowlicks]

context is this discussion on the discord

Feature Request

Swap usage of std::time with web_time where appropriate.
The wasm tracing_subscriber panics when you subscribe to FmtSpan::CLOSE events because they include span timing with std::time::Instant which isn't available on wasm.

Crates

tracing_subscriber

Motivation

Improve usage on wasm

Proposal

I'm not sure if it would be better to implement this with some #[cfg(target = "wasm32-unknown-unknown")] code, or if it should just be done across the board.

Alternatives

I don't know! I currently can't figure out a good way to measure the elapsed time of a span in wasm.