[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/config/config.go

@@ -55,6 +55,7 @@ func fieldtag(field, tag string) string {
 // https://github.com/mvdan/gofumpt.
 type Configuration struct {
 	LogLevel           string `name:"log-level" usage:"Log level to run at: [trace, debug, info, warn, fatal]"`
+	LogFormat          string `name:"log-format" usage:"Log output format: [logfmt, json]"`
 	LogTimestampFormat string `name:"log-timestamp-format" usage:"Format to use for the log timestamp, as supported by Go's time.Layout"`
 	LogDbQueries       bool   `name:"log-db-queries" usage:"Log database queries verbosely when log-level is trace or debug"`
 	LogClientIP        bool   `name:"log-client-ip" usage:"Include the client IP in logs"`

internal/config/defaults.go

@@ -29,6 +29,7 @@ import (
 // if you use this, you will still need to set Host, and, if desired, ConfigPath.
 var Defaults = Configuration{
 	LogLevel:           "info",
+	LogFormat:          "logfmt",
 	LogTimestampFormat: "02/01/2006 15:04:05.000",
 	LogDbQueries:       false,
 	ApplicationName:    "gotosocial",

internal/config/helpers.gen.go

@@ -30,6 +30,7 @@ import (
 
 const (
 	LogLevelFlag                                   = "log-level"
+	LogFormatFlag                                  = "log-format"
 	LogTimestampFormatFlag                         = "log-timestamp-format"
 	LogDbQueriesFlag                               = "log-db-queries"
 	LogClientIPFlag                                = "log-client-ip"
@@ -226,6 +227,7 @@ const (
 
 func (cfg *Configuration) RegisterFlags(flags *pflag.FlagSet) {
 	flags.String("log-level", cfg.LogLevel, "Log level to run at: [trace, debug, info, warn, fatal]")
+	flags.String("log-format", cfg.LogFormat, "Log output format: [logfmt, json]")
 	flags.String("log-timestamp-format", cfg.LogTimestampFormat, "Format to use for the log timestamp, as supported by Go's time.Layout")
 	flags.Bool("log-db-queries", cfg.LogDbQueries, "Log database queries verbosely when log-level is trace or debug")
 	flags.Bool("log-client-ip", cfg.LogClientIP, "Include the client IP in logs")
@@ -412,8 +414,9 @@ func (cfg *Configuration) RegisterFlags(flags *pflag.FlagSet) {
 }
 
 func (cfg *Configuration) MarshalMap() map[string]any {
-	cfgmap := make(map[string]any, 193)
+	cfgmap := make(map[string]any, 194)
 	cfgmap["log-level"] = cfg.LogLevel
+	cfgmap["log-format"] = cfg.LogFormat
 	cfgmap["log-timestamp-format"] = cfg.LogTimestampFormat
 	cfgmap["log-db-queries"] = cfg.LogDbQueries
 	cfgmap["log-client-ip"] = cfg.LogClientIP
@@ -623,6 +626,14 @@ func (cfg *Configuration) UnmarshalMap(cfgmap map[string]any) error {
 		}
 	}
 
+	if ival, ok := cfgmap["log-format"]; ok {
+		var err error
+		cfg.LogFormat, err = cast.ToStringE(ival)
+		if err != nil {
+			return fmt.Errorf("error casting %#v -> string for 'log-format': %w", ival, err)
+		}
+	}
+
 	if ival, ok := cfgmap["log-timestamp-format"]; ok {
 		var err error
 		cfg.LogTimestampFormat, err = cast.ToStringE(ival)
@@ -2218,6 +2229,28 @@ func GetLogLevel() string { return global.GetLogLevel() }
 // SetLogLevel safely sets the value for global configuration 'LogLevel' field
 func SetLogLevel(v string) { global.SetLogLevel(v) }
 
+// GetLogFormat safely fetches the Configuration value for state's 'LogFormat' field
+func (st *ConfigState) GetLogFormat() (v string) {
+	st.mutex.RLock()
+	v = st.config.LogFormat
+	st.mutex.RUnlock()
+	return
+}
+
+// SetLogFormat safely sets the Configuration value for state's 'LogFormat' field
+func (st *ConfigState) SetLogFormat(v string) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.LogFormat = v
+	st.reloadToViper()
+}
+
+// GetLogFormat safely fetches the value for global configuration 'LogFormat' field
+func GetLogFormat() string { return global.GetLogFormat() }
+
+// SetLogFormat safely sets the value for global configuration 'LogFormat' field
+func SetLogFormat(v string) { global.SetLogFormat(v) }
+
 // GetLogTimestampFormat safely fetches the Configuration value for state's 'LogTimestampFormat' field
 func (st *ConfigState) GetLogTimestampFormat() (v string) {
 	st.mutex.RLock()