mdbx: statistics of page operations.

Related to https://github.com/erthink/libmdbx/issues/186

Change-Id: Ia00e6e0df9a65f880517ca33e7f444a0526b96e1

mdbx.h

@@ -2239,6 +2239,23 @@ struct MDBX_envinfo {
   /** Current environment mode.
    * The same as \ref mdbx_env_get_flags() returns. */
   uint32_t mi_mode;
+
+  /** Statistics of page operations.
+   * \details Overall statistics of page operations of all (running, completed
+   * and aborted) transactions in the current multi-process session (since the
+   * first process opened the database). */
+  struct {
+    uint64_t newly;   /**< Quantity of a new pages added */
+    uint64_t cow;     /**< Quantity of pages copied for update */
+    uint64_t clone;   /**< Quantity of parent's dirty pages clones
+                           for nested transactions */
+    uint64_t split;   /**< Page splits */
+    uint64_t merge;   /**< Page merges */
+    uint64_t spill;   /**< Quantity of spilled dirty pages */
+    uint64_t unspill; /**< Quantity of unspilled/reloaded pages */
+    uint64_t wops;    /**< Number of explicit write operations (not a pages)
+                           to a disk */
+  } mi_pgop_stat;
 };
 #ifndef __cplusplus
 /** \ingroup c_statinfo */