refactor: reduce logger calls in hot path

```
                    │   old.txt   │              new.txt               │
                    │   sec/op    │   sec/op     vs base               │
Tailer_Tail/32-12     959.6µ ± 3%   704.0µ ± 2%  -26.64% (p=0.002 n=6)
Tailer_Tail/128-12    1.391m ± 2%   1.111m ± 1%  -20.14% (p=0.002 n=6)
Tailer_Tail/512-12    4.063m ± 1%   3.797m ± 2%   -6.54% (p=0.002 n=6)
Tailer_Tail/1024-12   7.874m ± 1%   7.637m ± 2%   -3.00% (p=0.002 n=6)
Tailer_Tail/4096-12   29.39m ± 1%   29.56m ± 3%        ~ (p=0.310 n=6)
geomean               4.166m        3.675m       -11.78%

                    │   old.txt    │               new.txt               │
                    │     B/s      │     B/s       vs base               │
Tailer_Tail/32-12     1.640Gi ± 3%   2.235Gi ± 2%  +36.30% (p=0.002 n=6)
Tailer_Tail/128-12    4.423Gi ± 2%   5.538Gi ± 1%  +25.21% (p=0.002 n=6)
Tailer_Tail/512-12    6.021Gi ± 1%   6.443Gi ± 2%   +7.00% (p=0.002 n=6)
Tailer_Tail/1024-12   6.208Gi ± 1%   6.400Gi ± 2%   +3.10% (p=0.002 n=6)
Tailer_Tail/4096-12   6.647Gi ± 1%   6.609Gi ± 3%        ~ (p=0.310 n=6)
geomean               4.479Gi        5.077Gi       +13.36%

                    │   old.txt    │               new.txt                │
                    │     B/op     │     B/op      vs base                │
Tailer_Tail/32-12     113.2Ki ± 0%   113.2Ki ± 0%       ~ (p=0.617 n=6)
Tailer_Tail/128-12    113.1Ki ± 0%   113.1Ki ± 0%       ~ (p=0.924 n=6)
Tailer_Tail/512-12    113.1Ki ± 0%   113.1Ki ± 0%       ~ (p=1.000 n=6)
Tailer_Tail/1024-12   113.1Ki ± 0%   113.1Ki ± 1%       ~ (p=1.000 n=6)
Tailer_Tail/4096-12   113.1Ki ± 0%   113.1Ki ± 0%       ~ (p=1.000 n=6) ¹
geomean               113.2Ki        113.2Ki       -0.00%
¹ all samples are equal

                    │  old.txt   │              new.txt               │
                    │ allocs/op  │ allocs/op   vs base                │
Tailer_Tail/32-12     16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
Tailer_Tail/128-12    16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
Tailer_Tail/512-12    16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
Tailer_Tail/1024-12   16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
Tailer_Tail/4096-12   16.00 ± 0%   16.00 ± 0%       ~ (p=1.000 n=6) ¹
geomean               16.00        16.00       +0.00%
¹ all samples are equal
```

tail.go

@@ -258,6 +258,7 @@ func (t *Tailer) Tail(ctx context.Context, h Handler) error {
 		done.Store(true)
 	}()
 
+	debugEnabled := t.lg.Core().Enabled(zapcore.DebugLevel)
 	for {
 		if done.Load() {
 			return ctx.Err()
@@ -266,17 +267,18 @@ func (t *Tailer) Tail(ctx context.Context, h Handler) error {
 		// Grab the offset in case we need to back up in the event of a half-line.
 		offset := t.offset()
 		line.Offset = offset
-		if e := t.lg.Check(zapcore.DebugLevel, "Offset"); e != nil {
-			e.Write(zap.Int64("offset", offset))
+		var readErr error
+		if debugEnabled {
+			t.lg.Debug("Reading line", zap.Int64("offset", offset))
 		}
 
-		var readErr error
-		t.lg.Debug("Reading line")
 		line.Data, readErr = t.readLine(line.Data)
 
 		switch readErr {
 		case io.EOF:
-			t.lg.Debug("Got EOF")
+			if debugEnabled {
+				t.lg.Debug("Got EOF")
+			}
 			if line.final() {
 				// Reporting only final lines, i.e. those ending with newline.
 				// Line can become final later.
@@ -297,7 +299,9 @@ func (t *Tailer) Tail(ctx context.Context, h Handler) error {
 				}
 				return nil
 			}
-			t.lg.Debug("Waiting for changes")
+			if debugEnabled {
+				t.lg.Debug("Waiting for changes")
+			}
 			if err := t.waitForChanges(ctx, offset); err != nil {
 				if errors.Is(err, errStop) {
 					return nil