Propose a metrics interface

include/ConflictSet.h

@@ -88,6 +88,32 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
   /** Returns the total bytes in use by this ConflictSet */
   int64_t getBytes() const;
 
+  /** Experimental! */
+  struct MetricsV1 {
+    /** A null-terminated string with static lifetime. Matches the regex
+     * [a-zA-Z_:][a-zA-Z0-9_:]*
+     */
+    const char *name;
+    /** A null-terminated string with static lifetime.  May contain any sequence
+     * of UTF-8 characters (after the metric name), but the backslash and the
+     * line feed characters have to be escaped as \\ and \n, respectively.
+     */
+    const char *help;
+    /** Counters are >= 0 and non-decreasing. Gauges are any value. */
+    enum Type { Counter, Gauge } type;
+    /** Thread-safety: do not read concurrently with a call to any non-const
+     * method in the ConflictSet associated with this metric. */
+    const double *value;
+  };
+
+  /** Experimental! Store a pointer to an array of MetricsV1 (owned by the
+   * ConflictSet) to `*metrics`, and its length to `*count`. This function makes
+   * no guarantees about the contents of the metrics (e.g. names, help text, and
+   * the meaning of values). A correct implementation is free to return nonsense
+   * or nothing at all. Not intended to be inspected programmatically. Only
+   * intended to be plumbed along to e.g. Prometheus. */
+  void getMetricsV1(MetricsV1 **metrics, int *count) const;
+
 #if __cplusplus > 199711L
   ConflictSet(ConflictSet &&) noexcept;
   ConflictSet &operator=(ConflictSet &&) noexcept;