🎨 Introduce contextualized logging

This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.

Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.

Closes #265

vendor/github.com/ipfs/go-log/v2/pipe.go

@@ -0,0 +1,90 @@
+package log
+
+import (
+	"io"
+
+	"go.uber.org/multierr"
+	"go.uber.org/zap/zapcore"
+)
+
+// A PipeReader is a reader that reads from the logger. It is synchronous
+// so blocking on read will affect logging performance.
+type PipeReader struct {
+	r      *io.PipeReader
+	closer io.Closer
+	core   zapcore.Core
+}
+
+// Read implements the standard Read interface
+func (p *PipeReader) Read(data []byte) (int, error) {
+	return p.r.Read(data)
+}
+
+// Close unregisters the reader from the logger.
+func (p *PipeReader) Close() error {
+	if p.core != nil {
+		loggerCore.DeleteCore(p.core)
+	}
+	return multierr.Append(p.core.Sync(), p.closer.Close())
+}
+
+// NewPipeReader creates a new in-memory reader that reads from all loggers
+// The caller must call Close on the returned reader when done.
+//
+// By default, it:
+//
+// 1. Logs JSON. This can be changed by passing the PipeFormat option.
+// 2. Logs everything that would otherwise be logged to the "primary" log
+//    output. That is, everything enabled by SetLogLevel. The minimum log level
+//    can be increased by passing the PipeLevel option.
+func NewPipeReader(opts ...PipeReaderOption) *PipeReader {
+	opt := pipeReaderOptions{
+		format: JSONOutput,
+		level:  LevelDebug,
+	}
+
+	for _, o := range opts {
+		o.setOption(&opt)
+	}
+
+	r, w := io.Pipe()
+
+	p := &PipeReader{
+		r:      r,
+		closer: w,
+		core:   newCore(opt.format, zapcore.AddSync(w), opt.level),
+	}
+
+	loggerCore.AddCore(p.core)
+
+	return p
+}
+
+type pipeReaderOptions struct {
+	format LogFormat
+	level  LogLevel
+}
+
+type PipeReaderOption interface {
+	setOption(*pipeReaderOptions)
+}
+
+type pipeReaderOptionFunc func(*pipeReaderOptions)
+
+func (p pipeReaderOptionFunc) setOption(o *pipeReaderOptions) {
+	p(o)
+}
+
+// PipeFormat sets the output format of the pipe reader
+func PipeFormat(format LogFormat) PipeReaderOption {
+	return pipeReaderOptionFunc(func(o *pipeReaderOptions) {
+		o.format = format
+	})
+}
+
+// PipeLevel sets the log level of logs sent to the pipe reader.
+func PipeLevel(level LogLevel) PipeReaderOption {
+	return pipeReaderOptionFunc(func(o *pipeReaderOptions) {
+		o.level = level
+	})
+}