Add support for counting invocations of now()

Bug: EW-7840
Test: internal

Author: ohodson

src/workerd/io/io-context.c++

@@ -773,6 +773,7 @@ size_t IoContext::getTimeoutCount() {
 
 kj::Date IoContext::now(IncomingRequest& incomingRequest) {
   kj::Date adjustedTime = incomingRequest.ioChannelFactory->getTimer().now();
+  incomingRequest.metrics->clockRead();
 
   KJ_IF_SOME (maybeNextTimeout, timeoutManager->getNextTimeout()) {
     // Don't return a time beyond when the next setTimeout() callback is intended to run. This

src/workerd/io/observer.h

@@ -70,6 +70,8 @@ class RequestObserver: public kj::Refcounted {
   virtual void finishedWaitUntilTask() {}
 
   virtual void setFailedOpen(bool value) {}
+
+  virtual uint64_t clockRead() { return 0; }
 };
 
 class IsolateObserver: public kj::AtomicRefcounted, public jsg::IsolateObserver {