Efficient Metric Recorder

Computing all metrics using constant extra memory.
PlotHistogram methods are executed in constant time.
-- Previously throughput and delay were using O(num_packets) extra memory and their associated PlotHistograms took linear time complexity.

Added MetricRecorder unittests

R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1257683006 .

Cr-Commit-Position: refs/heads/master@{#9658}