Avoid copying the histogram metric buckets. (jupp0r#331)

Author: dgavedissian

core/include/prometheus/histogram.h

@@ -57,7 +57,7 @@ class PROMETHEUS_CPP_CORE_EXPORT Histogram {
  /// Increments counters given a count for each bucket. (i.e. the caller of
  /// this function must have already sorted the values into buckets).
  /// Also increments the total sum of all observations by the given value.
- void ObserveMultiple(const std::vector<double> bucket_increments,
+ void ObserveMultiple(const std::vector<double>& bucket_increments,
  const double sum_of_values);
 
  /// \brief Get the current value of the counter.

core/src/histogram.cc

@@ -24,7 +24,7 @@ void Histogram::Observe(const double value) {
  bucket_counts_[bucket_index].Increment();
 }
 
-void Histogram::ObserveMultiple(const std::vector<double> bucket_increments,
+void Histogram::ObserveMultiple(const std::vector<double>& bucket_increments,
  const double sum_of_values) {
  if (bucket_increments.size() != bucket_counts_.size()) {
  throw std::length_error(