Support tracing_subscriber::fmt::FmtContext::new

[xu-cheng]

Feature Request

Crates

tracing-subscriber

Motivation

When implementing a custom tracing subscriber, I would like to reuse the existing tracing_subscriber::fmt::Format to format event messages. Although the tracing_subscriber::fmt::FormatEvent::format_event API is publicly available, it requires a tracing_subscriber::fmt::FmtContext as input. Currently, there is no way to construct a FmtContext outside the tracing-subscriber crate.

Proposal

Expose a public tracing_subscriber::fmt::FmtContext::new API.

Alternatives

Provide alternative public APIs that allow crate users to format tracing events into a String.