Expose XDP counters via Thrift API

Summary:
Expose the newly added XDP-level statistics counters through Katran's Thrift API

This builds on D82544156 which added the underlying BPF counters
(`XDP_TOTAL_CNTR`, `XDP_TX_CNTR`, `XDP_DROP_CNTR`, `XDP_PASS_CNTR`) by providing the necessary Thrift service methods

Reviewed By: lima1756, nikhildl12

Differential Revision: D82561662

fbshipit-source-id: e5f69786a642fa0db0b7b40584c31ef1cf6ae77e

Author: p-shah256

katran/lib/KatranLb.cpp

@@ -1948,6 +1948,22 @@ lb_stats KatranLb::getLruStats() {
   return getLbStats(config_.maxVips + kLruCntrOffset);
 }
 
+lb_stats KatranLb::getXdpTotalStats() {
+  return getLbStats(config_.maxVips + kXdpTotalOffset);
+}
+
+lb_stats KatranLb::getXdpTxStats() {
+  return getLbStats(config_.maxVips + kXdpTxOffset);
+}
+
+lb_stats KatranLb::getXdpDropStats() {
+  return getLbStats(config_.maxVips + kXdpDropOffset);
+}
+
+lb_stats KatranLb::getXdpPassStats() {
+  return getLbStats(config_.maxVips + kXdpPassOffset);
+}
+
 lb_stats KatranLb::getLruMissStats() {
   return getLbStats(config_.maxVips + kLruMissOffset);
 }

katran/lib/KatranLb.h

@@ -69,6 +69,10 @@ constexpr uint32_t kIcmpPtbV6Offset = 12;
 constexpr uint32_t kIcmpPtbV4Offset = 13;
 constexpr uint32_t kXPopDecapSuccessfulOffset = 14;
 constexpr uint32_t kUdpFlowMigrationInvalidationOffset = 15;
+constexpr uint32_t kXdpTotalOffset = 16;
+constexpr uint32_t kXdpTxOffset = 17;
+constexpr uint32_t kXdpDropOffset = 18;
+constexpr uint32_t kXdpPassOffset = 19;
 
 /**
  * LRU map related constants
@@ -525,6 +529,38 @@ class KatranLb {
    */
   lb_stats getLruStats();
 
+  /**
+   * @return struct lb_stats w/ statistics
+   * for packets + bytes "touched" by lb
+   *
+   * helper function which returns total amount
+   */
+  lb_stats getXdpTotalStats();
+
+  /**
+   * @return struct lb_stats w/ statistics
+   * for packets + bytes forwarded to vip backends
+   *
+   * helper function which returns total amount
+   */
+  lb_stats getXdpTxStats();
+
+  /**
+   * @return struct lb_stats w/ statistics
+   * for packets + bytes dropped by lb
+   *
+   * helper function which returns total amount
+   */
+  lb_stats getXdpDropStats();
+
+  /**
+   * @return struct lb_stats w/ statistics
+   * for packets + bytes passed up to the kernel
+   *
+   * helper function which returns total amount
+   */
+  lb_stats getXdpPassStats();
+
   /**
    * @return struct lb_stats w/ statistic of the reasons for lru misses
    *