datadog-formatting-layer 1.0.1

# Datadog Formatting Layer

A crate providing a tracing-subscriber layer for formatting events so Datadog can parse them.

## Features

- Provides a layer for tracing-subscriber
- Generates parsable "logs" for datadog and prints them to stdout
- Enables log correlation between spans and "logs" (see [datadog docs](https://docs.datadoghq.com/tracing/other_telemetry/connect_logs_and_traces/))

## Why not just `tracing_subscriber::fmt().json()` ?

The problem is, that datadog expects the "logs" to be in a specific (mostly undocumented) json format.

This crates tries to mimic this format.

## Usage

### Simple

```rust
use datadog_formatting_layer::DatadogFormattingLayer;
use tracing::info;
use tracing_subscriber::prelude::*;

tracing_subscriber::registry()
    .with(DatadogFormattingLayer)
    .init();

info!(user = "Jack", "Hello World!");
```

Running this code will result in the following output on stdout:

```json
{
  "timestamp": "2023-06-21T10:36:50.364874878+00:00",
  "level": "INFO",
  "message": "Hello World user=Jack",
  "target": "simple"
}
```

### With Opentelemetry

```rust
use datadog_formatting_layer::DatadogFormattingLayer;
use opentelemetry::{
    global,
    sdk::{
        propagation::TraceContextPropagator,
        trace::{RandomIdGenerator, Sampler},
    },
};
use opentelemetry_datadog::ApiVersion;
use tracing::{debug, error, info, instrument, warn};
use tracing_subscriber::{prelude::*, util::SubscriberInitExt};

// Just some otel boilerplate
global::set_text_map_propagator(TraceContextPropagator::new());

let tracer = opentelemetry_datadog::new_pipeline()
    .with_service_name("my-service")
    .with_trace_config(
        opentelemetry::sdk::trace::config()
            .with_sampler(Sampler::AlwaysOn)
            .with_id_generator(RandomIdGenerator::default()),
    )
    .with_api_version(ApiVersion::Version05)
    .with_env("rls")
    .with_version("420")
    .install_simple()
    .unwrap();

// Use both the tracer and the formatting layer
tracing_subscriber::registry()
    .with(DatadogFormattingLayer)
    .with(tracing_opentelemetry::layer().with_tracer(tracer))
    .init();

// Here no span exists
info!(user = "Jack", "Hello World!");
some_test("fasel");

// This will create a span and a trace id which is attached to the "logs"
#[instrument(fields(hello = "world"))]
fn some_test(value: &str) {
    // Here some span exists
    info!(ola = "salve", value, "Bla {value}");
}
```

When running this code with an datadog agent installed the logs will be sent to datadog
and parsed there.

Otherwise the following output will be printed to stdout

```json
{"timestamp":"2023-06-21T10:36:50.363224217+00:00","level":"INFO","message":"Hello World! user=Jack","target":"otel"}
{"timestamp":"2023-06-21T10:36:50.363384118+00:00","level":"INFO","message":"Bla fasel user=Jack ola=salve value=Fasel hello=world","target":"otel","dd.trace_id":0,"dd.span_id":10201226522570980512}
```