Make metrics thread-safe

Even concurrently with calling non-const methods on the associated
ConflictSet

include/ConflictSet.h

@@ -101,9 +101,10 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
     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;
+    /** Get the most up-to-date value available for this metric. Thread-safe. */
+    double getValue() const;
+    /** @private */
+    void *p;
   };
 
   /** Experimental! Store a pointer to an array of MetricsV1 (owned by the
@@ -111,7 +112,8 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
    * 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. */
+   * intended to be plumbed along to e.g. Prometheus. Callers may repeatedly
+   * call `getValue` on the metrics they're interested in. */
   void getMetricsV1(MetricsV1 **metrics, int *count) const;
 
 #if __cplusplus > 199711L