(new): add log level helpers and HTTP method levels

- move LogLevel and predefined levels into levels.go
- add level-specific constructors for info/warn/error/fatal/debug
- add predefined HTTP method log levels and LogLevelForMethod
- unify Logger Print/Println internals and add Printf
- improve GoDoc coverage for exported APIs

formatter.go

@@ -6,16 +6,28 @@ import (
 	"time"
 )
 
-// Formatter
-// Format: %t - time, %N - name, %l - level, %L - level uppercase, %b - traceback, %B - full traceback, %m - message,
-// %M - method, %f - filename, %n - line number, %s - method signature, %p - full path
-// Example: "%t [%l] %N: %m (%f:%n %s)"
+// Formatter configures how log records are rendered.
 //
-// TimeStampFormat: %Y - year, %m - month, %d - day, %H - hour, %M - minute, %S - second,
-// %z - timezone(i.e. 0300), %Z - timezone(i.e. 03:00)
-// Example: "%d.%m.%Y %H:%M:%S"
+// Format tokens:
+//   - %t: time
+//   - %N: name
+//   - %l: level
+//   - %L: uppercase level
+//   - %b: first traceback frame
+//   - %B: full traceback
+//   - %m: message
+//   - %M: method
+//   - %f: filename
+//   - %n: line number
+//   - %s: method signature
+//   - %p: full path
 //
-// TraceBackFormat: %M - method, %f - filename, %n - line number, %s - method signature, %p - full path
+// Example format: "%t [%l] %N: %m (%f:%n %s)"
+//
+// TimeStampFormat uses strftime-like directives such as %Y, %m, %d, %H, %M,
+// %S, %z, and %Z.
+//
+// TraceBackFormat supports the traceback-related tokens %M, %f, %n, %s, and %p.
 type Formatter struct {
 	Format           string
 	MessageSeparator string
@@ -31,6 +43,7 @@ type Formatter struct {
 	ColorOnlyStdout bool
 }
 
+// DefaultTextFormatter is the default formatter used by text writers.
 var DefaultTextFormatter = &Formatter{
 	Format:             "%t %l %N: %m",
 	MessageSeparator:   " ",
@@ -40,6 +53,8 @@ var DefaultTextFormatter = &Formatter{
 	ColorOutput:        true,
 	ColorOnlyStdout:    true,
 }
+
+// DefaultJsonFormatter is the default formatter used by JSON writers.
 var DefaultJsonFormatter = &Formatter{
 	Format:           "%m",
 	MessageSeparator: " ",