fix(flutter/android): surface getMetrics failures into the in-app log

Traced the whole traffic-counter pipeline (Dart -> MethodChannel ->
Kotlin -> JNI -> Bridge) end to end; it's architecturally identical to
the working desktop implementation, so no code-level bug was found.
Previously a getMetrics exception was only reported as a PlatformException
that Dart swallows with a bare debugPrint, invisible in the in-app log
viewer users actually have access to. Now it's also written to the
native log buffer via OstpClientSdk.addLog, so if the counter breaks
again the actual cause (exception vs. genuinely-zero atomics) shows up
in View Logs instead of requiring adb.

ostp-flutter/android/app/src/main/kotlin/com/ospab/ostp_client/MainActivity.kt

@@ -92,6 +92,13 @@ class MainActivity : FlutterActivity() {
                         val metrics = net.ostp.client.OstpClientSdk.getMetrics()
                         result.success(metrics ?: "{}")
                     } catch (e: Throwable) {
+                        // Surfaced into the in-app log viewer (not just logcat) so a
+                        // broken traffic counter is diagnosable from a user's bug
+                        // report without adb access.
+                        android.util.Log.e("MainActivity", "getMetrics failed", e)
+                        try {
+                            net.ostp.client.OstpClientSdk.addLog("getMetrics failed: ${e.javaClass.simpleName}: ${e.message}")
+                        } catch (_: Throwable) {}
                         result.error("ERROR", e.message, null)
                     }
                 }