### Go slog-logstash Basic Usage Example

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Demonstrates how to initialize and use the `slog-logstash` handler with a TCP connection, including examples of logging errors and structured information with groups and attributes.

```go
import (
    pool "github.com/samber/go-tcp-pool"
    sloglogstash "github.com/samber/slog-logstash/v2"
    "log/slog"
)

func main() {
    // ncat -l 9999 -k
    conn, err := pool.Dial("tcp", "localhost:9999")
    if err != nil {
        log.Fatal(err)
    }

    logger := slog.New(sloglogstash.Option{Level: slog.LevelDebug, Conn: conn}.NewLogstashHandler())
    logger = logger.
        With("environment", "dev").
        With("release", "v1.0.0")

    // log error
    logger.
        With("category", "sql").
        With("query.statement", "SELECT COUNT(*) FROM users;").
        With("query.duration", 1*time.Second).
        With("error", fmt.Errorf("could not count users")).
        Error("caramba!")

    // log user signup
    logger.
        With(
            slog.Group("user",
                slog.String("id", "user-123"),
                slog.Time("created_at", time.Now()),
            ),
        ).
        Info("user registration")
}
```

--------------------------------

### Install slog-logstash Go Module

Source: https://github.com/samber/slog-logstash/blob/main/README.md

This command fetches and installs the `slog-logstash` Go module into your project. It requires Go version 1.21 or higher for compatibility.

```Shell
go get github.com/samber/slog-logstash/v2
```

--------------------------------

### Example JSON Output from slog-logstash

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Illustrates the JSON payload structure generated by `slog-logstash` for both error and info level logs, showing how attributes, groups, and timestamps are formatted for Logstash.

```json
{
    "@timestamp":"2023-04-10T14:00:0.000000+00:00",
    "level":"ERROR",
    "message":"caramba!",
    "error":{
        "error":"could not count users",
        "kind":"*errors.errorString",
        "stack":null
    },
    "extra":{
        "environment":"dev",
        "release":"v1.0.0",
        "category":"sql",
        "query.statement":"SELECT COUNT(*) FROM users;",
        "query.duration": "1s"
    }
}


{
    "@timestamp":"2023-04-10T14:00:0.000000+00:00",
    "level":"INFO",
    "message":"user registration",
    "error":null,
    "extra":{
        "environment":"dev",
        "release":"v1.0.0",
        "user":{
            "id":"user-123",
            "created_at":"2023-04-10T14:00:0.000000+00:00"
        }
    }
}
```

--------------------------------

### Development and Testing Commands for slog-logstash

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Provides common `make` commands for setting up development dependencies and running tests for the `slog-logstash` project.

```bash
# Install some dev dependencies
make tools

# Run tests
make test
# or
make watch-test
```

--------------------------------

### Integrate OpenTelemetry Tracing with slog-logstash in Go

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Shows how to integrate `slog-logstash` with OpenTelemetry to extract trace and span IDs from the context and include them as attributes in log entries.

```go
import (
	sloglogstash "github.com/samber/slog-logstash"
	slogotel "github.com/samber/slog-otel"
	"go.opentelemetry.io/otel/sdk/trace"
)

func main() {
	tp := trace.NewTracerProvider(
		trace.WithSampler(trace.AlwaysSample()),
	)
	tracer := tp.Tracer("hello/world")

	ctx, span := tracer.Start(context.Background(), "foo")
	defer span.End()

	span.AddEvent("bar")

	logger := slog.New(
		sloglogstash.Option{
			// ...
			AttrFromContext: []func(ctx context.Context) []slog.Attr{
				slogotel.ExtractOtelAttrFromContext([]string{"tracing"}, "trace_id", "span_id"),
			},
		}.NewLogstashHandler(),
	)

	logger.ErrorContext(ctx, "a message")
}
```

--------------------------------

### Configure Global slog-logstash Parameters in Go

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Sets global configuration parameters for `slog-logstash`, such as keys for source, context, and error attributes in the log payload.

```go
sloglogstash.SourceKey = "source"
sloglogstash.ContextKey = "extra"
sloglogstash.ErrorKeys = []string{"error", "err"}
```

--------------------------------

### Define slog-logstash Handler Options in Go

Source: https://github.com/samber/slog-logstash/blob/main/README.md

Defines the `Option` struct used to configure the `slog-logstash` handler, including log level, connection, custom converters, and attribute extraction from context.

```go
type Option struct {
	// log level (default: debug)
	Level slog.Leveler

	// connection to logstash
	Conn net.Conn

	// optional: customize json payload builder
	Converter Converter
	// optional: custom marshaler
	Marshaler func(v any) ([]byte, error)
	// optional: fetch attributes from context
	AttrFromContext []func(ctx context.Context) []slog.Attr

	// optional: see slog.HandlerOptions
	AddSource   bool
	ReplaceAttr func(groups []string, a slog.Attr) slog.Attr
}
```

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