[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/logfmt.go

@@ -0,0 +1,67 @@
+// 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 (
+	"time"
+
+	"code.superseriousbusiness.org/gotosocial/internal/log/level"
+	"codeberg.org/gruf/go-byteutil"
+	"codeberg.org/gruf/go-caller"
+	"codeberg.org/gruf/go-kv/v2"
+	"codeberg.org/gruf/go-kv/v2/format"
+)
+
+var args = format.DefaultArgs()
+
+type Logfmt struct{ Base }
+
+func (fmt *Logfmt) Format(buf *byteutil.Buffer, stamp time.Time, pc uintptr, lvl level.LEVEL, kvs []kv.Field, msg string) {
+	if fmt.TimeFormat != "" {
+		// Append formatted timestamp string.
+		buf.B = append(buf.B, `timestamp="`...)
+		fmt.AppendFormatStamp(buf, stamp)
+		buf.B = append(buf.B, `" `...)
+	}
+
+	// Append formatted calling func.
+	buf.B = append(buf.B, `func=`...)
+	buf.B = append(buf.B, caller.Get(pc)...)
+	buf.B = append(buf.B, ' ')
+
+	if lvl != level.UNSET {
+		// Append formatted level string.
+		buf.B = append(buf.B, `level=`...)
+		buf.B = append(buf.B, lvl.String()...)
+		buf.B = append(buf.B, ' ')
+	}
+
+	// Append formatted fields.
+	for _, field := range kvs {
+		kv.AppendQuoteString(buf, field.K)
+		buf.B = append(buf.B, '=')
+		buf.B = format.Global.Append(buf.B, field.V, args)
+		buf.B = append(buf.B, ' ')
+	}
+
+	if msg != "" {
+		// Append formatted msg string.
+		buf.B = append(buf.B, `msg=`...)
+		kv.AppendQuoteString(buf, msg)
+	}
+}