Implement LOG_FORMAT=json (#173)

Centralized log collection system works better with logs in json format.
E.g. DataDog strongly encourage setting up your logging library to produce your logs in JSON format to avoid the need for custom parsing rules.
So, the next small fix is all we need to get json logs.

Signed-off-by: Sergey Belyaev <sbelyaev@setronica.com>

README.md

@@ -19,6 +19,7 @@
       - [Example 3](#example-3)
       - [Example 4](#example-4)
   - [Loading Configuration](#loading-configuration)
+  - [Log Format](#log-format)
 - [Request Fields](#request-fields)
 - [Statistics](#statistics)
 - [HTTP Port](#http-port)
@@ -347,6 +348,40 @@ The former is the default behavior. To use the latter method, set the `RUNTIME_W
 
 For more information on how runtime works you can read its [README](https://github.com/lyft/goruntime).
 
+## Log Format
+
+A centralized log collection system works better with logs in json format. JSON format avoids the need for custom parsing rules.
+The Ratelimit service produces logs in a text format by default. For Example:
+
+```
+time="2020-09-10T17:22:35Z" level=debug msg="loading domain: messaging"
+time="2020-09-10T17:22:35Z" level=debug msg="loading descriptor: key=messaging.message_type_marketing"
+time="2020-09-10T17:22:35Z" level=debug msg="loading descriptor: key=messaging.message_type_marketing.to_number ratelimit={requests_per_unit=5, unit=DAY}"
+time="2020-09-10T17:22:35Z" level=debug msg="loading descriptor: key=messaging.to_number ratelimit={requests_per_unit=100, unit=DAY}"
+time="2020-09-10T17:21:55Z" level=warning msg="Listening for debug on ':6070'"
+time="2020-09-10T17:21:55Z" level=warning msg="Listening for HTTP on ':8080'"
+time="2020-09-10T17:21:55Z" level=debug msg="waiting for runtime update"
+time="2020-09-10T17:21:55Z" level=warning msg="Listening for gRPC on ':8081'"
+```
+
+JSON Log format can be configured using the following environment variables:
+
+```
+LOG_FORMAT=json
+```
+
+Output example:
+```
+{"@message":"loading domain: messaging","@timestamp":"2020-09-10T17:22:44.926010192Z","level":"debug"}
+{"@message":"loading descriptor: key=messaging.message_type_marketing","@timestamp":"2020-09-10T17:22:44.926019315Z","level":"debug"}
+{"@message":"loading descriptor: key=messaging.message_type_marketing.to_number ratelimit={requests_per_unit=5, unit=DAY}","@timestamp":"2020-09-10T17:22:44.926037174Z","level":"debug"}
+{"@message":"loading descriptor: key=messaging.to_number ratelimit={requests_per_unit=100, unit=DAY}","@timestamp":"2020-09-10T17:22:44.926048993Z","level":"debug"}
+{"@message":"Listening for debug on ':6070'","@timestamp":"2020-09-10T17:22:44.926113905Z","level":"warning"}
+{"@message":"Listening for gRPC on ':8081'","@timestamp":"2020-09-10T17:22:44.926182006Z","level":"warning"}
+{"@message":"Listening for HTTP on ':8080'","@timestamp":"2020-09-10T17:22:44.926227031Z","level":"warning"}
+{"@message":"waiting for runtime update","@timestamp":"2020-09-10T17:22:44.926267808Z","level":"debug"}
+```
+
 # Request Fields
 
 For information on the fields of a Ratelimit gRPC request please read the information

src/service_cmd/runner/runner.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"math/rand"
 	"net/http"
+	"strings"
 	"time"
 
 	stats "github.com/lyft/gostats"
@@ -43,6 +44,16 @@ func (runner *Runner) Run() {
 	} else {
 		logger.SetLevel(logLevel)
 	}
+	if strings.ToLower(s.LogFormat) == "json" {
+		logger.SetFormatter(&logger.JSONFormatter{
+			TimestampFormat: time.RFC3339Nano,
+			FieldMap: logger.FieldMap{
+				logger.FieldKeyTime: "@timestamp",
+				logger.FieldKeyMsg:  "@message",
+			},
+		})
+	}
+
 	var localCache *freecache.Cache
 	if s.LocalCacheSizeInBytes != 0 {
 		localCache = freecache.NewCache(s.LocalCacheSizeInBytes)

src/settings/settings.go

@@ -22,6 +22,7 @@ type Settings struct {
 	RuntimeIgnoreDotFiles        bool          `envconfig:"RUNTIME_IGNOREDOTFILES" default:"false"`
 	RuntimeWatchRoot             bool          `envconfig:"RUNTIME_WATCH_ROOT" default:"true"`
 	LogLevel                     string        `envconfig:"LOG_LEVEL" default:"WARN"`
+	LogFormat                    string        `envconfig:"LOG_FORMAT" default:"text"`
 	RedisSocketType              string        `envconfig:"REDIS_SOCKET_TYPE" default:"unix"`
 	RedisUrl                     string        `envconfig:"REDIS_URL" default:"/var/run/nutcracker/ratelimit.sock"`
 	RedisPoolSize                int           `envconfig:"REDIS_POOL_SIZE" default:"10"`