PLT-3893: Structured Logging Continues (#7252)

* PLT-3893: Imported logger work from https://github.com/MusikPolice/platform

* PLT-3893: Integrated logger with system config

* PLT-3893: Integrated Mattermost config with logging solution, modified log message serialization so entire message is serialized as a JSON object

* PLT-3893: Added support for format strings in Debug methods. Added an overload that does not require a Context object for cases when one isn't available

* PLT-3893: Added context and format string support to debug and error methods

* PLT-3893: A few updates from pull request feedback

* PLT-3893: Changed tests to use testify

* Fixed TestAddRemoveConfigListener to no longer assume that there are zero config listeners when the test begins, since other tests could add config listeners

* Updated TestGetDeletedChannelsForTeam so that it doesn't assume state when it begins

* PLT-3893: Changed File property of log message so that it's relative to /mattermost directory, rather than to wherever the user is running the application from on their machine

* Flipped expected/actual assert arguments, added an explicit test for getCallerFilename(...), since it's failing on Jenkins

* Added printlns to debug failing tests on Jenkins

* Relaxed test cases to avoid failure on Jenkins caused by code coverage calculations. Removed printlns.

* Changed the way that caller filename is determined to make it more robust, updated tests to make them more lax, while not choking on the strange paths that Jenkins uses.

* Fixed gofmt issues

* Added debug output to tests to diagnose Jenkins build failures

* Still trying to get some useful debug logging on Jenkins

* Changed getCallerFilename to handle the strange paths that runtime.Caller(...) returns on Jenkins

* Fixing checkstyle issues

utils/logger/log4go_json_writer.go

@@ -0,0 +1,27 @@
+// glue functions that allow logger.go to leverage log4Go to write JSON-formatted log records to a file
+
+package logger
+
+import (
+	l4g "github.com/alecthomas/log4go"
+	"github.com/mattermost/platform/utils"
+)
+
+// newJSONLogWriter is a utility method for creating a FileLogWriter set up to
+// output JSON record log messages instead of line-based ones.
+func newJSONLogWriter(fname string, rotate bool) *l4g.FileLogWriter {
+	return l4g.NewFileLogWriter(fname, rotate).SetFormat(
+		`{"level": "%L", 
+		  "timestamp": "%D %T", 
+		  "source": "%S", 
+		  "message": %M 
+		}`).SetRotateLines(utils.LOG_ROTATE_SIZE)
+}
+
+// NewJSONFileLogger - Create a new logger with a "file" filter configured to send JSON-formatted log messages at
+// or above lvl to a file with the specified filename.
+func NewJSONFileLogger(lvl l4g.Level, filename string) l4g.Logger {
+	return l4g.Logger{
+		"file": &l4g.Filter{Level: lvl, LogWriter: newJSONLogWriter(filename, true)},
+	}
+}