Add X-Request-ID to HttpHeaders.

Fixes #5306.

RELNOTES=Add `X-Request-ID` to `HttpHeaders`.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=341479875

Author: TiagoGrosso

android/guava/src/com/google/common/net/HttpHeaders.java

@@ -418,6 +418,12 @@ private ReferrerPolicyValues() {}
    * @since 15.0
    */
   @Beta public static final String PUBLIC_KEY_PINS_REPORT_ONLY = "Public-Key-Pins-Report-Only";
+  /**
+   * The HTTP {@code X-Request-ID} header field name.
+   *
+   * @since NEXT
+   */
+  public static final String X_REQUEST_ID = "X-Request-ID";
   /** The HTTP {@code X-Requested-With} header field name. */
   public static final String X_REQUESTED_WITH = "X-Requested-With";
   /** The HTTP {@code X-User-IP} header field name. */

guava/src/com/google/common/net/HttpHeaders.java

@@ -418,6 +418,12 @@ private ReferrerPolicyValues() {}
    * @since 15.0
    */
   @Beta public static final String PUBLIC_KEY_PINS_REPORT_ONLY = "Public-Key-Pins-Report-Only";
+  /**
+   * The HTTP {@code X-Request-ID} header field name.
+   *
+   * @since NEXT
+   */
+  public static final String X_REQUEST_ID = "X-Request-ID";
   /** The HTTP {@code X-Requested-With} header field name. */
   public static final String X_REQUESTED_WITH = "X-Requested-With";
   /** The HTTP {@code X-User-IP} header field name. */