tracing_subscriber: add API to display tracing module path

[gdesmott]

Feature Request

Crates

tracing_subscriber

Motivation

tracing-gstreamer populate the module path with the C function name from the C API. I'd like to display it in my log but it looks like tracing_subscriber does not have API to do so.

[hawkw]

It should be fairly straightforward to add a new toggle to the Format type, here https://github.com/tokio-rs/tracing/blob/2f6fcb0fabff541d14b0fe3a5211b8f472fec4cb/tracing-subscriber/src/fmt/format/mod.rs#L251-L260 and extend the existing formatters to display the module path when configured to do so. For example, we would add something similar to this in the default formatter: https://github.com/tokio-rs/tracing/blob/2f6fcb0fabff541d14b0fe3a5211b8f472fec4cb/tracing-subscriber/src/fmt/format/mod.rs#L606-L608 except we would display the value of meta.module_path() rather than meta.target().

If you're interested in working on a PR to add this, I'm happy to answer any questions; otherwise, I'll see about adding it when I have some spare time.

One minor design question that does come to mind, though: for most events and spans emitted through the tracing macros, the the target and module_path metadata values will be the same, since the target defaults to the module path when not explicitly overridden. If the formatter is configured to display both targets and module paths, we may want to skip printing both when these values are equal; otherwise, the majority of events would just print the module path twice.