json log (#1137)

Add BAML_LOG_JSON environment variable to enable logging baml_events and
other logs as json blobs. Serialize all the data as json.

<!-- ELLIPSIS_HIDDEN -->


----

> [!IMPORTANT]
> Add JSON logging to BAML server using `tracing` and
`tracing-subscriber`, configurable via environment variable.
> 
>   - **Behavior**:
> - Add JSON logging support using `tracing` and `tracing-subscriber` in
`baml-runtime` and `cli`.
> - JSON logging can be enabled via `BAML_LOG_JSON` environment
variable.
>     - Logs include detailed event and metadata information.
>   - **Configuration**:
> - Update `Cargo.toml` and `Cargo.lock` to include `tracing`,
`tracing-subscriber`, and related dependencies.
>     - Add `rustflags` for `tracing_unstable` in `.cargo/config.toml`.
>   - **Logging**:
> - Implement `Visualize` trait for structured logging in
`tracing/mod.rs`.
> - Add `BamlEventJson` and `TokenUsage` structs for JSON log
serialization.
> - Modify `main.rs` in `cli` to initialize logging based on environment
variable.
>   - **Misc**:
>     - Comment out unused `println` in `old_mod.rs`.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 46d35c5. It will automatically
update as commits are pushed.</sup>


<!-- ELLIPSIS_HIDDEN -->

engine/.cargo/config.toml

@@ -1,3 +1,7 @@
+# to enable json logging we need this serialization feaure. https://docs.rs/tracing-subscriber/latest/tracing_subscriber/#unstable-features
+[build]
+rustflags = ["--cfg", "tracing_unstable"]
+
 # https://github.com/rust-lang/cargo/issues/8607
 [target.x86_64-unknown-linux-musl]
 rustflags = ["-C", "target-feature=-crt-static"]

engine/baml-lib/jsonish/src/jsonish/old/old_mod.rs

@@ -144,7 +144,7 @@ impl Target {
         match res {
             Ok(value) => {
                 if !state.is_empty() {
-                    println!("{:?}", state);
+                    // println!("{:?}", state);
                 }
                 Ok(value)
             }

engine/baml-runtime/Cargo.toml

@@ -86,6 +86,9 @@ ambassador = "0.4.0"
 aws-smithy-json = "0.60.7"
 jsonwebtoken = "9.3.0"
 pretty_assertions = "1.4.0"
+valuable = { version = "0.1.0", features = ["derive"] }
+tracing = { version = "0.1.40", features = ["valuable"] }
+tracing-subscriber = { version = "0.3.18", features = ["json", "env-filter","valuable"] }
 
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
@@ -128,11 +131,16 @@ reqwest.workspace = true
 walkdir = "2.5.0"
 which = "6.0.3"
 
+
+
+
 [features]
 defaults = []
 internal = []
 skip-integ-tests = []
 
+
+
 [dev-dependencies]
 assert_cmd = "2"
 console_log = "1"

engine/baml-runtime/src/internal/llm_client/mod.rs

@@ -268,6 +268,7 @@ pub struct LLMCompleteResponseMetadata {
     pub total_tokens: Option<u64>,
 }
 
+// This is how the response gets logged if you print the result to the console.
 impl std::fmt::Display for LLMCompleteResponse {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         writeln!(
@@ -289,17 +290,26 @@ impl std::fmt::Display for LLMCompleteResponse {
     }
 }
 
+// This is the one that gets logged by BAML_LOG, for baml_events log.
 impl crate::tracing::Visualize for LLMCompleteResponse {
     fn visualize(&self, max_chunk_size: usize) -> String {
         let s = vec![
             format!(
                 "{}",
                 format!(
-                    "Client: {} ({}) - {}ms. StopReason: {}",
+                    "Client: {} ({}) - {}ms. StopReason: {}. Tokens(in/out): {}/{}",
                     self.client,
                     self.model,
                     self.latency.as_millis(),
-                    self.metadata.finish_reason.as_deref().unwrap_or("unknown")
+                    self.metadata.finish_reason.as_deref().unwrap_or("unknown"),
+                    self.metadata
+                        .prompt_tokens
+                        .map(|t| t.to_string())
+                        .unwrap_or_else(|| "unknown".to_string()),
+                    self.metadata
+                        .output_tokens
+                        .map(|t| t.to_string())
+                        .unwrap_or_else(|| "unknown".to_string()),
                 )
                 .yellow()
             ),