[core] improve metric agent connection log (#57056)

The log emitted during the metric agent connection is currently noisy
because it runs in a loop. Only emit the initialization log once. Keep
the log about connection established the same; the log timestamp will
give us the information we need regarding how long the connection takes.

Test:
- CI

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Reduce log spam by logging "Initializing exporter ..." only once at
the start of the retry loop in
`MetricsAgentClientImpl::WaitForServerReadyWithRetry`.
> 
> - **Logging**:
> - In `src/ray/rpc/metrics_agent_client.cc`, update
`MetricsAgentClientImpl::WaitForServerReadyWithRetry` to log
`"Initializing exporter ..."` only when `retry_count == 0` (first retry
attempt), reducing repeated logs during retries.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
23ece57. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Signed-off-by: Cuong Nguyen <[email protected]>

Author: can-anyscale

src/ray/rpc/metrics_agent_client.cc

@@ -37,7 +37,10 @@ void MetricsAgentClientImpl::WaitForServerReadyWithRetry(
     return;
   }
 
-  RAY_LOG(INFO) << "Initializing exporter ...";
+  if (retry_count == 0) {
+    // Only log the first time we start the retry loop.
+    RAY_LOG(INFO) << "Initializing exporter ...";
+  }
   HealthCheck(rpc::HealthCheckRequest(),
               [this, init_exporter_fn](auto &status, auto &&reply) {
                 if (status.ok() && !exporter_initialized_) {