chore: updated AGENTS.md with more testing guidelines

.agents/AGENTS.md

@@ -9,3 +9,38 @@
   Status / StatusOr checks (using `google::cloud::testing_util::IsOk`).
 - Use `EXPECT_TRUE` / `ASSERT_TRUE` (or `EXPECT_FALSE` / `ASSERT_FALSE`) for
   standard boolean expression checks (e.g., `stream.good()`).
+- **Prefer Declarative Container Matchers Over Imperative Loops:** Avoid manual
+  loops (`for (...)`), boolean search flags (`bool found = false;`), and manual
+  container filtering in tests. Use GoogleTest container matchers (e.g.,
+  `testing::Contains`, `testing::Each`, `testing::ElementsAre`,
+  `testing::UnorderedElementsAre`, `testing::IsEmpty`, `testing::SizeIs`) to
+  express collection assertions declaratively.
+- **Write Custom Matchers for Complex Objects and Protobufs:** When validating
+  complex objects, structs, or protobuf messages (e.g., time series, spans,
+  requests, responses), define custom matchers using `MATCHER_P` / `MATCHER_P2`
+  with `ExplainMatchResult`. Compose them using `testing::AllOf`,
+  `testing::AnyOf`, `testing::Property`, and `testing::Field`.
+  - *Why:* Declarative matchers provide detailed diagnostic mismatch
+    explanations when tests fail, whereas boolean flags only output
+    `Value of: found, Actual: false, Expected: true`.
+- **Example Pattern:**
+  ```cpp
+  MATCHER_P(MetricType, matcher, "") {
+    return ExplainMatchResult(matcher, arg.metric().type(), result_listener);
+  }
+
+  MATCHER_P2(HasMetricLabel, key, val_matcher, "") {
+    auto const& labels = arg.metric().labels();
+    auto it = labels.find(key);
+    if (it == labels.end()) {
+      *result_listener << "no metric label '" << key << "'";
+      return false;
+    }
+    return ExplainMatchResult(val_matcher, it->second, result_listener);
+  }
+
+  // Composed assertion:
+  EXPECT_THAT(recorded_metrics, Contains(AllOf(
+      MetricType(HasSubstr("outstanding_rpcs")),
+      HasMetricLabel("channel_pool_lb_policy", Eq("RANDOM_TWO_LEAST_USED")))));
+  ```