Adding a way to inject a request ID (#1046)

* Adding a way to inject a request ID

It is very useful to associate a request ID to each incoming request,
this change allows to provide a function to do that via Server Option.
The change comes with a default function which will generate a new
request ID. The request ID is put in the request context along with a
common logger which always logs the request-id

We add gRPC interceptors to the server so it can get the request ID out
of the gRPC metadata and put it in the common logger stored in the
context so as all the log lines using the common logger from the context
will have the request ID logged

api/common/request_id_util.go

@@ -0,0 +1,21 @@
+package common
+
+import (
+	"context"
+
+	"github.com/fnproject/fn/api/id"
+)
+
+// FnRequestID returns the passed value if that is not empty otherwise it generates a new unique ID
+func FnRequestID(ridFound string) string {
+	if ridFound == "" {
+		return id.New().String()
+	}
+	return ridFound
+}
+
+//RequestIDFromContext extract the request id from the context
+func RequestIDFromContext(ctx context.Context) string {
+	rid, _ := ctx.Value(contextKey(RequestIDContextKey)).(string)
+	return rid
+}