add: readme for slog (#63)

* add: readme for slog

* fix: slog readme code indentation

log/slog/README.md

@@ -0,0 +1,88 @@
+# Slog
+
+Slog (short for "structured log") is a wrapper over [go-kit/log](https://github.com/go-kit/log) with a defined log structure and a set of helper methods.
+
+
+## Configurations
+
+| Environment Variable | Default Value   | Allowed Values   |
+| --- | :---: | :---: |
+| LOG_LEVEL | "info"   | "debug", "info", "warn", "error"   |
+| LOG_CALLER_DEPTH | 4   | Z |
+
+
+## Log Levels & Filtering
+
+All log levels are allowed to be logged by default. Use the config "LOG_LEVEL" to filter out.
+
+| Set Value | |
+| ---   | --- |
+| "debug" | debug + info + warn + error |
+| "info" | info + warn + error |
+| "warn" | warn + error |
+| "error" | error |
+
+
+## Usage
+
+1. Log line without custom fields:
+```
+slog.Debug("the five boxing")
+```
+```
+level=debug ts=2011-08-22T10:07:34.329181992Z caller=main.go:21 msg="the five boxing"
+```
+
+2. Log line with custom fields:
+```
+slog.Warn("wizards jump quickly", "unit_bytes", 8008)
+```
+```
+level=warn ts=2012-07-18T11:27:32.616223846Z caller=main.go:70 msg="the five boxing" unit_bytes=8008
+```
+
+3. Create a logger with trace_id attached
+```
+slogger := slog.WithTraceId(ctx)
+slogger.Info("the quick brown")
+```
+
+4. Create a logger with custom fields attached
+```
+logFields := map[string]interface{}{
+	"integration_uuid": response.IntegrationUuid,
+}
+slogger := slog.WithFields(logFields)
+slogger.Warn("fox jumps over")
+```
+
+5. Create a logger with trace_id and custom fields attached
+```
+logFields := map[string]interface{}{
+	"integration_uuid": response.IntegrationUuid,
+}
+slogger := slog.WithFields(logFields).WithTraceId(ctx)
+slogger.Debug("the lazy dog")
+```
+
+
+## Logging Methods
+```
+slog.Debug(msg string, args ...any)
+slog.Debugf(format string, args ...any)
+slog.Info(msg string, args ...any)
+slog.Infof(format string, args ...any)
+slog.Warn(msg string, args ...any)
+slog.Warnf(format string, args ...any)
+slog.Error(err error, msg string, args ...any)
+slog.Errorf(err error, format string, args ...any)
+```
+
+
+## Helper Methods
+
+```
+slog.WithFields(fields map[string]any) Logger
+slog.WithTraceId(ctx context.Context) Logger
+slog.DefaultLogger() log.Logger
+```