[feature + performance] add JSON logging format (#4355)

# Description

Adds JSON logging as an optional alternative log output format. In the process this moves our log formatting itself into a separate subpkg to make it more easily modular, and improves caller name getting with some calling function name caching.

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4355
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/log/format/format.go

@@ -0,0 +1,90 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package format
+
+import (
+	"sync/atomic"
+	"time"
+
+	"code.superseriousbusiness.org/gotosocial/internal/log/level"
+	"codeberg.org/gruf/go-byteutil"
+	"codeberg.org/gruf/go-kv/v2"
+)
+
+var (
+	// ensure func signature conformance.
+	_ FormatFunc = (*Logfmt)(nil).Format
+	_ FormatFunc = (*JSON)(nil).Format
+)
+
+// FormatFunc defines a function capable of formatting a log entry (args = 1+) to a given buffer (args = 0).
+type FormatFunc func(buf *byteutil.Buffer, stamp time.Time, pc uintptr, lvl level.LEVEL, kvs []kv.Field, msg string) //nolint:revive
+
+type Base struct {
+	// TimeFormat defines time.Format() layout to
+	// use when appending a timestamp to log entry.
+	TimeFormat string
+
+	// stampCache caches recently formatted stamps.
+	//
+	// see the following benchmark:
+	// goos: linux
+	// goarch: amd64
+	// pkg: code.superseriousbusiness.org/gotosocial/internal/log/format
+	// cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
+	// BenchmarkStampCache
+	// BenchmarkStampCache-16          272199975                4.447 ns/op           0 B/op          0 allocs/op
+	// BenchmarkNoStampCache
+	// BenchmarkNoStampCache-16        76041058                15.94 ns/op            0 B/op          0 allocs/op
+	stampCache atomic.Pointer[struct {
+		stamp  time.Time
+		format string
+	}]
+}
+
+// AppendFormatStamp will append given timestamp according to TimeFormat,
+// caching recently formatted stamp strings to reduce number of Format() calls.
+func (b *Base) AppendFormatStamp(buf *byteutil.Buffer, stamp time.Time) {
+	const precision = time.Millisecond
+
+	// Load cached stamp value.
+	last := b.stampCache.Load()
+
+	// Round stamp to min precision.
+	stamp = stamp.Round(precision)
+
+	// If a cached entry exists use this string.
+	if last != nil && stamp.Equal(last.stamp) {
+		buf.B = append(buf.B, last.format...)
+		return
+	}
+
+	// Else format new and store ASAP,
+	// i.e. ignoring any CAS result.
+	format := stamp.Format(b.TimeFormat)
+	b.stampCache.CompareAndSwap(last, &struct {
+		stamp  time.Time
+		format string
+	}{
+		stamp:  stamp,
+		format: format,
+	})
+
+	// Finally, append new timestamp.
+	buf.B = append(buf.B, format...)
+}