Add a new format, JSON_GCP (#61)

When deploying a service that sends out logs to cloud logging, the
severity was not parsed and because logs of our services are all sent
out of STDERR, they all defaulted to `severity: "ERROR"`, making it
quite hard to read.

This PRs adds a new `SRC_LOG_FORMAT=json_gcp`, which outputs json logs
in a structure that cloud logging understands.

See the results of a little test in Cloud run: 


![image](https://github.com/sourcegraph/log/assets/10151/88b81ae5-936f-4f63-a22e-7ad7796151e2)

init.go

@@ -14,7 +14,7 @@ var (
 	// EnvLogFormat is key of the environment variable that is used to set the log format
 	// on Init.
 	//
-	// The value should be one of 'json' or 'console', defaulting to 'json'.
+	// The value should be one of 'json', 'json_gcp' or 'condensed', defaulting to 'json'.
 	EnvLogFormat = "SRC_LOG_FORMAT"
 	// EnvLogLevel is key of the environment variable that can be used to set the log
 	// level on Init.

internal/encoders/config.go

@@ -38,6 +38,31 @@ var OpenTelemetryConfig = zapcore.EncoderConfig{
 	EncodeCaller:   zapcore.ShortCallerEncoder,
 }
 
+var GCPConfig = zapcore.EncoderConfig{
+	NameKey: "InstrumentationScope",
+	// https://cloud.google.com/logging/docs/agent/logging/configuration#timestamp-processing
+	TimeKey:    "timestampNanos",
+	EncodeTime: zapcore.EpochNanosTimeEncoder,
+	// https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#logseverity
+	// https://cloud.google.com/logging/docs/agent/logging/configuration#special-fields
+	LevelKey:    "severity",
+	EncodeLevel: zapcore.CapitalLevelEncoder, // most levels correspond to the OT level text
+	// https://cloud.google.com/logging/docs/agent/logging/configuration#special-fields
+	MessageKey: "message",
+
+	// These don't really have an equivalent in the OT spec, and we can't stick it under
+	// Attributes because they are top-level traits in Zap, so we just capitalize them and
+	// hope for the best.
+	CallerKey:     "Caller",
+	FunctionKey:   "Function",
+	StacktraceKey: "Stacktrace",
+
+	// Defaults
+	LineEnding:     zapcore.DefaultLineEnding,
+	EncodeDuration: zapcore.SecondsDurationEncoder,
+	EncodeCaller:   zapcore.ShortCallerEncoder,
+}
+
 // applyDevConfig applies options for dev environments to the encoder config
 func applyDevConfig(cfg zapcore.EncoderConfig) zapcore.EncoderConfig {
 	// Nice colors based on log level
@@ -75,6 +100,8 @@ func BuildEncoder(format output.Format, development bool) (enc zapcore.Encoder)
 		return zapcore.NewConsoleEncoder(config)
 	case output.FormatJSON:
 		return zapcore.NewJSONEncoder(config)
+	case output.FormatJSONGCP:
+		return zapcore.NewJSONEncoder(GCPConfig)
 	default:
 		panic("unknown output format")
 	}

output/output.go

@@ -7,6 +7,10 @@ const (
 	// FormatJSON encodes log entries to a machine-readable, OpenTelemetry-structured
 	// format.
 	FormatJSON Format = "json"
+	// FormatJSONGCP encodes log entries to a machine-readable, GCP-structured format.
+	// It's similar to OpenTelemetry-structured format, but the severity field
+	// complies with https://cloud.google.com/logging/docs/structured-logging#special-payload-fields
+	FormatJSONGCP Format = "json_gcp"
 	// FormatConsole encodes log entries to a human-readable format.
 	FormatConsole Format = "console"
 )
@@ -16,16 +20,18 @@ const (
 // log formats.
 func ParseFormat(format string) Format {
 	switch format {
-	case string(FormatJSON),
-		// True 'logfmt' has significant limitations around certain field types:
-		// https://github.com/jsternberg/zap-logfmt#limitations so since it implies a
-		// desire for a somewhat structured format, we interpret it as OutputJSON.
-		"logfmt":
+	case string(FormatJSONGCP):
+		return FormatJSONGCP
+
+	// True 'logfmt' has significant limitations around certain field types:
+	// https://github.com/jsternberg/zap-logfmt#limitations so since it implies a
+	// desire for a somewhat structured format, we interpret it as OutputJSON.
+	case string(FormatJSON), "logfmt":
 		return FormatJSON
-	case string(FormatConsole),
-		// The previous 'condensed' format is optimized for local dev, so it serves the
-		// same purpose as OutputConsole
-		"condensed":
+
+	// The previous 'condensed' format is optimized for local dev, so it serves the
+	// same purpose as OutputConsole
+	case string(FormatConsole), "condensed":
 		return FormatConsole
 	}