tracing: generate macros tests

There are a large number of combinations of field types which currently are not
matched by the level event macros (e.g. `info!`).

Some recent examples from #3407 is the following that doesn't work:

```rust
info!(name: "order.received.ok", order.id = 123, "order received");
```

However, the corresponding `event!` macro does work:

```rust
event!(name: "order.received.ok", Level::INFO, order.id = 123, "order received");
```

And other variants also work:

```rust
// Without `name:` directive
info!(order.id = 123, "order received");

// With another field before the dotted one
info!(name: "order.received.ok", foo = true, order.id = 123, "order received");
```

Many such cases have been fixed in the past (#2983, #2883, #2879). However,
this has been a bit like wack-a-mole, where we keep finding issues and then
fixing those issues, adding extra tests for them and then going on our way.

Since the complexity is often in combinations (as above, only when using the
`name:` directive together with a dotted field name on the first field), it
would be useful to have some extensive tests that attempt to cover all possible
combinations.

It turns out that there are **a lot** of combiantions.

This change adds an `xtask` that generates tests for event macros (span macros
are out of scope for this change) similar to the ones found in
`tracing/tests/macros.rs`. Which is to say, tests containing macros which
should compile, but don't need to run.

Owing to the large number of combinations, the tests are split into test files
by macro (e.g. `event!`, `info!`) and directive combination (e.g. no
directives, just `name:`, `name:` and `target:`). The tests are kept in a
separate crate outside the main workspace to avoid rust-analyzer trying to
parse the files - as they are quite large. Specifically, there are 1220 macro
invocations per test file.

The test files are generated (no need to check them in) and then validated by
`cargo check` on CI.

When run against the unmodified macros in `tracing`, there were 504 `info!`
macro invocations which failed to compile, out of a total of 9760 (including
all combinations of directives).

Author: hds

Cargo.toml

@@ -16,8 +16,8 @@ members = [
     "tracing-test",
     "tracing-appender",
     "tracing-journald",
-    "examples"
-]
+    "examples",
+    "xtask"]
 
  # This will be ignored with Rust older than 1.74, but for now that's okay;
  # we're only using it to fix check-cfg issues that first appeared in Rust 1.80.

tracing/src/macros.rs

@@ -0,0 +1,10 @@
+[workspace]
+
+[package]
+name = "test-macros"
+version = "0.1.0"
+publish = false
+edition = "2018"
+
+[dependencies]
+tracing = { path = ".." }

tracing/test-macros/Cargo.toml

@@ -0,0 +1 @@
+*

tracing/test-macros/tests/.gitignore

@@ -0,0 +1,9 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+
+[lints]
+workspace = true