Fix: prevent race in slog text handler group handling

Closes #50450.

This updates our custom slog text handler to take heavier
inspiration from the slog.TextHandler. A variant of handleState
used by the slog.TextHandler has been vendored and modified to
produce the same output as our custom logrus formatter. Offloading
formatting from the SlogTextHandler directly to handleState prevents
the race caused in #50450.

Additionally, some quality of life improvements were added by
moving some code around to reduce file sizes and better define
what belongs in a file.

Benchmarks indicate that the changes here don't move the needle
much.

```
goos: darwin
goarch: arm64
pkg: github.com/gravitational/teleport/lib/utils/log
cpu: Apple M2 Pro
                               │   old.txt    │              new.txt               │
                               │    sec/op    │   sec/op     vs base               │
Formatter/logrus/text-12         8.665µ ± 16%   8.187µ ± 9%       ~ (p=0.280 n=10)
Formatter/logrus/json-12         8.879µ ±  2%   8.820µ ± 1%       ~ (p=0.089 n=10)
Formatter/slog/default_text-12   3.936µ ±  3%   3.946µ ± 4%       ~ (p=0.839 n=10)
Formatter/slog/text-12           3.789µ ±  2%   3.431µ ± 1%  -9.45% (p=0.000 n=10)
Formatter/slog/default_json-12   3.005µ ±  4%   3.032µ ± 3%       ~ (p=0.739 n=10)
Formatter/slog/json-12           3.029µ ±  6%   3.022µ ± 1%       ~ (p=0.381 n=10)
geomean                          4.675µ         4.557µ       -2.52%

                               │   old.txt    │                new.txt                │
                               │     B/op     │     B/op      vs base                 │
Formatter/logrus/text-12         5.936Ki ± 0%   5.936Ki ± 0%       ~ (p=0.752 n=10)
Formatter/logrus/json-12         6.212Ki ± 0%   6.211Ki ± 0%       ~ (p=0.752 n=10)
Formatter/slog/default_text-12   2.534Ki ± 0%   2.534Ki ± 0%       ~ (p=1.000 n=10) ¹
Formatter/slog/text-12           2.144Ki ± 0%   2.167Ki ± 0%  +1.09% (p=0.000 n=10)
Formatter/slog/default_json-12   2.448Ki ± 0%   2.448Ki ± 0%       ~ (p=1.000 n=10) ¹
Formatter/slog/json-12           2.318Ki ± 0%   2.318Ki ± 0%       ~ (p=1.000 n=10) ¹
geomean                          3.231Ki        3.236Ki       +0.18%
¹ all samples are equal

                               │  old.txt   │               new.txt                │
                               │ allocs/op  │ allocs/op   vs base                  │
Formatter/logrus/text-12         54.00 ± 0%   54.00 ± 0%        ~ (p=1.000 n=10) ¹
Formatter/logrus/json-12         76.00 ± 0%   76.00 ± 0%        ~ (p=1.000 n=10) ¹
Formatter/slog/default_text-12   41.00 ± 0%   41.00 ± 0%        ~ (p=1.000 n=10) ¹
Formatter/slog/text-12           52.00 ± 0%   33.00 ± 0%  -36.54% (p=0.000 n=10)
Formatter/slog/default_json-12   41.00 ± 0%   41.00 ± 0%        ~ (p=1.000 n=10) ¹
Formatter/slog/json-12           42.00 ± 0%   42.00 ± 0%        ~ (p=1.000 n=10) ¹
geomean                          49.70        46.07        -7.30%
¹ all samples are equal

```

lib/utils/log/buffer.go

@@ -21,6 +21,14 @@ func newBuffer() *buffer {
 	return bufPool.Get().(*buffer)
 }
 
+func (b *buffer) Len() int {
+	return len(*b)
+}
+
+func (b *buffer) SetLen(n int) {
+	*b = (*b)[:n]
+}
+
 func (b *buffer) Free() {
 	// To reduce peak allocation, return only smaller buffers to the pool.
 	const maxBufferSize = 16 << 10
@@ -49,35 +57,6 @@ func (b *buffer) WriteByte(c byte) error {
 	return nil
 }
 
-func (b *buffer) WritePosInt(i int) {
-	b.WritePosIntWidth(i, 0)
-}
-
-// WritePosIntWidth writes non-negative integer i to the buffer, padded on the left
-// by zeroes to the given width. Use a width of 0 to omit padding.
-func (b *buffer) WritePosIntWidth(i, width int) {
-	// Cheap integer to fixed-width decimal ASCII.
-	// Copied from log/log.go.
-
-	if i < 0 {
-		panic("negative int")
-	}
-
-	// Assemble decimal in reverse order.
-	var bb [20]byte
-	bp := len(bb) - 1
-	for i >= 10 || width > 1 {
-		width--
-		q := i / 10
-		bb[bp] = byte('0' + i - q*10)
-		bp--
-		i = q
-	}
-	// i < 10
-	bb[bp] = byte('0' + i)
-	b.Write(bb[bp:])
-}
-
 func (b *buffer) String() string {
 	return string(*b)
 }

lib/utils/log/formatter_test.go

@@ -76,7 +76,6 @@ func TestOutput(t *testing.T) {
 	loc, err := time.LoadLocation("Africa/Cairo")
 	require.NoError(t, err, "failed getting timezone")
 	clock := clockwork.NewFakeClockAt(time.Now().In(loc))
-	formattedNow := clock.Now().UTC().Format(time.RFC3339)
 
 	t.Run("text", func(t *testing.T) {
 		// fieldsRegex matches all the key value pairs emitted after the message and before the caller. All fields are
@@ -88,7 +87,7 @@ func TestOutput(t *testing.T) {
 		// 2) the message
 		// 3) the fields
 		// 4) the caller
-		outputRegex := regexp.MustCompile("(\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}Z)(\\s+.*)(\".*diag_addr`\\.\")(.*)(\\slog/formatter_test.go:\\d{3})")
+		outputRegex := regexp.MustCompile("(\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d{3}Z)(\\s+.*)(\".*diag_addr`\\.\")(.*)(\\slog/formatter_test.go:\\d{3})")
 
 		tests := []struct {
 			name        string
@@ -149,7 +148,7 @@ func TestOutput(t *testing.T) {
 					EnableColors: true,
 					ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
 						if a.Key == slog.TimeKey {
-							a.Value = slog.StringValue(formattedNow)
+							a.Value = slog.TimeValue(clock.Now().UTC())
 						}
 						return a
 					},
@@ -461,7 +460,13 @@ func TestConcurrentOutput(t *testing.T) {
 			wg.Add(1)
 			go func(i int) {
 				defer wg.Done()
-				logger.InfoContext(ctx, "Teleport component entered degraded state", "component", i)
+				logger.InfoContext(ctx, "Teleport component entered degraded state",
+					"component", i,
+					slog.Group("group",
+						slog.String("test", "123"),
+						slog.String("animal", "llama"),
+					),
+				)
 			}(i)
 		}
 		wg.Wait()