Add initial unit test coverage for Metrics event persistence

[firasrg]

What

Adds the first unit test for the analytics system.

• Added MetricsTests to verify Metrics#count(...) persists a metric event;
• Asserted both the stored event name and the recorded timestamp;
• Added async waiting/timeout handling for reliable verification of background writes;
• Exposed the Metrics executor service so tests can shut it down cleanly;
• Enabled debug logging for org.togetherjava.tjbot in test runs;
• Added AssertJ for test assertions

Why

Metrics performs asynchronous database writes, so having direct test coverage here improves confidence in analytics persistence and makes future refactors safer.

Validation

• ran :application:test --tests org.togetherjava.tjbot.features.MetricsTests

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Zabuzard]

undo

[Zabuzard]

undo. we want to use raw junit in this project for now as it is easier for beginners and reflects more what they will see in the wild and might already be familiar with from their own projects and university etc

[Zabuzard]

undo, this won't be needed (see other comment why)

[Zabuzard]

this test is too much in terms of complexity for no tangible benefit.

lets step back and create something simpler that beginners can understand and maintain while providing pretty much the same test-safety in terms of relevant coverage.

1. create a package private method in Metrics called countBlocking. its identical to count but it won't use the executor service. so it creates the instant and then calls processEvent directly
2. in ur unit test this will be the method ur testing against now. without the async stuff the test will simplify a lot
3. the test should use a GWT structure (see other examples in the codebase, alternative patterns that share the same idea: 4-phases SEVT or AAA). try to aim for something like this:

@Test
void basicEventCount() {
  // GIVEN a test event
  String expectedEvent = "test";
  Instant expectedHappenedAt = Instant.now();

  // WHEN counting the event
  metrics.countBlocking(expectedEvent);

  // THEN the event was saved in the DB
  Foo entry = database...
  String actualEvent = entry.event();
  Instant actualHappenedAt = entry.happenedAt();

  assertEquals(expectedEvent, actualEvent);
  assertCloseEnough(expectedHappenedAt, actualHappenedAt);
} 

private static void assertCloseEnough(Instant expected, Instant actual) {
 ... // assert that its within 1min difference
} 

(double check the correct order of expected vs actual in assert, i never can remember it, lol)