chore(x-goog-spanner-request-id): commit foundation

This change adds in the bases to bring in the functionality
for the "x-goog-spanner-request-id" that'll be used for debugging
without the limits of trace continuity and sampling.

Updates googleapis#3537

Address code review comments

Author: odeke-em

google-cloud-spanner/src/main/java/com/google/cloud/spanner/XGoogSpannerRequestId.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.cloud.spanner;
+
+import com.google.api.core.InternalApi;
+import com.google.common.annotations.VisibleForTesting;
+import java.security.SecureRandom;
+import java.util.Objects;
+
+@InternalApi
+public class XGoogSpannerRequestId {
+  // 1. Generate the random process Id singleton.
+  @VisibleForTesting static String RAND_PROCESS_ID = XGoogSpannerRequestId.generateRandProcessId();
+
+  @VisibleForTesting
+  static final long VERSION = 1; // The version of the specification being implemented.
+
+  private final long nthClientId;
+  private final long nthChannelId;
+  private final long nthRequest;
+  private long attempt;
+
+  XGoogSpannerRequestId(long nthClientId, long nthChannelId, long nthRequest, long attempt) {
+    this.nthClientId = nthClientId;
+    this.nthChannelId = nthChannelId;
+    this.nthRequest = nthRequest;
+    this.attempt = attempt;
+  }
+
+  public static XGoogSpannerRequestId of(
+      long nthClientId, long nthChannelId, long nthRequest, long attempt) {
+    return new XGoogSpannerRequestId(nthClientId, nthChannelId, nthRequest, attempt);
+  }
+
+  private static String generateRandProcessId() {
+    return String.format("%08x", new SecureRandom().nextInt());
+  }
+
+  @Override
+  public String toString() {
+    return String.format(
+        "%d.%s.%d.%d.%d.%d",
+        XGoogSpannerRequestId.VERSION,
+        XGoogSpannerRequestId.RAND_PROCESS_ID,
+        this.nthClientId,
+        this.nthChannelId,
+        this.nthRequest,
+        this.attempt);
+  }
+
+  @Override
+  public boolean equals(Object other) {
+    // instanceof for a null object returns false.
+    if (!(other instanceof XGoogSpannerRequestId)) {
+      return false;
+    }
+
+    XGoogSpannerRequestId otherReqId = (XGoogSpannerRequestId) (other);
+
+    return Objects.equals(this.nthClientId, otherReqId.nthClientId)
+        && Objects.equals(this.nthChannelId, otherReqId.nthChannelId)
+        && Objects.equals(this.nthRequest, otherReqId.nthRequest)
+        && Objects.equals(this.attempt, otherReqId.attempt);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(this.nthClientId, this.nthChannelId, this.nthRequest, this.attempt);
+  }
+}

google-cloud-spanner/src/test/java/com/google/cloud/spanner/XGoogSpannerRequestIdTest.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.cloud.spanner;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class XGoogSpannerRequestIdTest {
+  private static final Pattern REGEX_RAND_PROCESS_ID =
+      Pattern.compile("1.([0-9a-z]{8})(\\.\\d+){3}\\.(\\d+)$");
+
+  @Test
+  public void testEquals() {
+    XGoogSpannerRequestId reqID1 = XGoogSpannerRequestId.of(1, 1, 1, 1);
+    XGoogSpannerRequestId reqID2 = XGoogSpannerRequestId.of(1, 1, 1, 1);
+    assertEquals(reqID1, reqID2);
+    assertEquals(reqID1, reqID1);
+    assertEquals(reqID2, reqID2);
+
+    XGoogSpannerRequestId reqID3 = XGoogSpannerRequestId.of(1, 1, 1, 2);
+    assertNotEquals(reqID1, reqID3);
+    assertNotEquals(reqID3, reqID1);
+    assertEquals(reqID3, reqID3);
+  }
+
+  @Test
+  public void testEnsureHexadecimalFormatForRandProcessID() {
+    String str = XGoogSpannerRequestId.of(1, 2, 3, 4).toString();
+    Matcher m = XGoogSpannerRequestIdTest.REGEX_RAND_PROCESS_ID.matcher(str);
+    assertTrue(m.matches());
+  }
+}