Metrics - Add histograms/README.md

base/metrics/histogram_macros.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_METRICS_HISTOGRAM_MACROS_H_
 #define BASE_METRICS_HISTOGRAM_MACROS_H_
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_macros_internal.h"
 #include "base/metrics/histogram_macros_local.h"
 #include "base/time/time.h"
 
+// Macros for efficient use of histograms.
+//
+// For best practices on deciding when to emit to a histogram and what form
+// the histogram should take, see tools/metrics/histograms/README.md
+//
 // TODO(nikunjb): Move sparse macros to this file.
+//
 // UMA_HISTOGRAM_SPARSE_SLOWLY is defined in sparse_histogram.h as it has
 // different #include dependencies.
 
 // TODO(rkaplow): Link to proper documentation on metric creation once we have
 // it in a good state.
 
 // All of these macros must be called with |name| as a runtime constant - it
 // doesn't have to literally be a constant, but it must be the same string on
 // all calls from a particular call site. If this rule is violated, it is
 // possible the data will be written to the wrong histogram.
@@ skipping 223 matching lines @@
 // |custom_ranges|. See comments above CustomRanges::FactoryGet about the
 // requirement of |custom_ranges|. You can use the helper function
 // CustomHistogram::ArrayToCustomRanges to transform a C-style array of valid
 // sample values to a std::vector<int>.
 #define UMA_HISTOGRAM_CUSTOM_ENUMERATION(name, sample, custom_ranges)          \
     STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample),                          \
         base::CustomHistogram::FactoryGet(name, custom_ranges,                 \
             base::HistogramBase::kUmaTargetedHistogramFlag))
 
 #endif  // BASE_METRICS_HISTOGRAM_MACROS_H_