Support structured logging (KV)

[SpriteOvO]

Implementation of #70, working in progress.

Steps

• Add kv::Key and kv::Value
  • Let's try using value-bag directly (alias) for value first.
• Add kvs field to struct Record.
• Logging macros accepts kv optional parameter.
  • I plan to use this syntax

    info!(kv: { a = 1, b = 2 }, "payload {}", 123);
    info!("payload {}", 123, kv: { a = 1, b = 2 });

    The kv is optional, just like the existing logger argument, and I may wish to allow them to be in any order. The colon allows us to distinguish with the formatting arguments so that it can be placed anywhere.
  • We can consider whether the following more advanced usages will be supported in this PR or in the future, but these syntaxes have not yet been decided.

    let key = "a";
    info!(kv: { {key} = 1, b = 2 }, "payload {}", 123);
    
    let kvs = HashMap::from_iter([("a", 1), ("b", 2)]);
    info!(kv: kvs, "payload {}", 123);

• Add tests.
• Support it in FullFormatter.
• Support it in PatternFormatter.
  • How to design placeholders for KV? - Simply adding a new {kv} pattern to output like "{ k1=1, k2=text }"
  • Change to "k1=1 k2=text", separated by whitespace, and brackets are not added by default (users can easily opt-in to add brackets themselves)
• Serialize KVs to a separated kv field in JsonFormatter.
• Key modifier in macros.

  info!(kv: { k1: = value, k2:? = value }, "message"); // "message { k1=display, k2=debug }"

• Shorthand in macros.

  let num = 123;
  info!(kv: { num }, "message"); // "message { num=123 }"

• Support parameter arbitrary ordering. (might require to refactor it in proc-macro)
• Try to replace slice with phf map for KV. (self-reminder: benchmarking it..)
  • phf only supports limited types for key (doc), and we have a custom type kv::Key, so abandon the idea.
• Add benchmarks.
• Support conversion from log crate's KV.
  • Blocked by Derive Clone for kv::Value rust-lang/log#668
    Merged, waiting for a release. Prepare for 0.4.26 release rust-lang/log#670
  • Benchmarks, is it slower than before?; Yes, but hard to optimize, because of the API designs from log crate.
• Support for capturing values by sval and serde.
• Add documentation and examples (macros, accessing in Record, downcasting (see doc of value-bag), etc.).
• Bump MSRV to v1.61 from v1.60,
  • Bump dependency sval_buffer to v2.3 sval-rs/value-bag#97
    Merged, waiting for a release.

[SpriteOvO]

I think this PR is ready. @Lancern Would you like to take a look? Any concerns or considerations?

[Copilot]

Copilot reviewed 32 out of 32 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

spdlog/src/formatter/journald_formatter.rs:52

• Consider adding a separator between the payload and the key-values (e.g. a whitespace) to ensure the log output is clearly delimited.

record.key_values().write_to(dest, true)?;

[SpriteOvO]

Just found and tried the new GitHub feature - requesting reviews from Copilot, but the results is a false-positive :(