Remove call to isatty from the hot path.

The result from isatty is immutable during the run of the program and
is triggering a heavy system call that significantly slow down answering
request.

$ benchstat before.txt after.txt
goos: darwin
goarch: amd64
pkg: github.com/gin-contrib/logger
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
         │  before.txt   │              after.txt               │
         │    sec/op     │    sec/op     vs base                │
Logger      2.924µ ±  2%   1.617µ ± 12%  -44.70% (p=0.000 n=10)
Logger-2   1649.0n ±  2%   766.1n ±  1%  -53.54% (p=0.000 n=10)
Logger-4   1009.5n ± 11%   496.6n ± 12%  -50.80% (p=0.000 n=10)
Logger-8    853.4n ±  3%   396.4n ± 12%  -53.56% (p=0.000 n=10)
geomean     1.428µ         702.7n        -50.78%

logger.go

@@ -32,6 +32,8 @@ type config struct {
 	serverErrorLevel zerolog.Level
 }
 
+var isTerm bool = isatty.IsTerminal(os.Stdout.Fd())
+
 // SetLogger initializes the logging middleware.
 func SetLogger(opts ...Option) gin.HandlerFunc {
 	cfg := &config{
@@ -56,7 +58,6 @@ func SetLogger(opts ...Option) gin.HandlerFunc {
 	}
 
 	return func(c *gin.Context) {
-		isTerm := isatty.IsTerminal(os.Stdout.Fd())
 		l := zerolog.New(cfg.output).
 			Output(
 				zerolog.ConsoleWriter{