Fix Context Values are not logged.

Only the Sentry hook uses the values of the passed context. Therefore, we removed the values from our log statements when we shifted them from an extra `WithField` call to the context.
We fix this behavior by introducing a Logrus Hook that copies a fixed set of context values to the logging data.

pkg/logging/context_hook.go

@@ -0,0 +1,41 @@
+package logging
+
+import (
+	"github.com/openHPI/poseidon/pkg/dto"
+	"github.com/sirupsen/logrus"
+)
+
+// ContextHook logs the values referenced by the of dto.LoggedContextKeys.
+// By default Logrus does not log the values stored in the passed context.
+type ContextHook struct{}
+
+// Fire is triggered on new log entries.
+func (hook *ContextHook) Fire(entry *logrus.Entry) error {
+	if entry.Context != nil {
+		injectContextValuesIntoData(entry)
+	}
+	return nil
+}
+
+func injectContextValuesIntoData(entry *logrus.Entry) {
+	for _, key := range dto.LoggedContextKeys {
+		value := entry.Context.Value(key)
+		_, valueExisting := entry.Data[string(key)]
+		if !valueExisting && value != nil {
+			entry.Data[string(key)] = value
+		}
+	}
+}
+
+// Levels returns all levels this hook should be registered to.
+func (hook *ContextHook) Levels() []logrus.Level {
+	return []logrus.Level{
+		logrus.PanicLevel,
+		logrus.FatalLevel,
+		logrus.ErrorLevel,
+		logrus.WarnLevel,
+		logrus.InfoLevel,
+		logrus.DebugLevel,
+		logrus.TraceLevel,
+	}
+}