api/ops: Add missing MSG_OP_GENERIC_USDT to OpCodeStrings

The MSG_OP_GENERIC_USDT opcode (28) was defined but missing from the
OpCodeStrings map. This would cause the String() method to return
"Unknown(28)" and log a bug report message when this opcode was
encountered.

Signed-off-by: Tam Mach <[email protected]>

Author: sayboras

docs/content/en/docs/reference/metrics.md

@@ -80,6 +80,7 @@ var OpCodeStrings = map[OpCode]string{
 	MSG_OP_CGROUP:             "Cgroup",
 	MSG_OP_LOADER:             "Loader",
 	MSG_OP_THROTTLE:           "Throttle",
+	MSG_OP_GENERIC_USDT:       "GenericUSDT",
 	MSG_OP_TEST:               "Test",
 }