feat(agent): emit metainfo download time and throughput

[Anton-Kalpakchiev]

Before every P2P download in an agent, the agent must first acquire the metainfo of that torrent. That process has shown to sometimes be a massive bottleneck in P2P downloads, as origin must download the whole blob from remote storage (e.g. GCS) in order to calculate its metainfo.

This PR adds client-side metrics (i.e. in agent) to record the latency and throughput of the metainfo request. Metrics are tagged by blob size, just like e2e torrent downloads in agent are.

Before this PR, the metainfo latency was already emitted, but

1. that latency was not tagged by torrent_size (which is relevant to put the latency into context)
2. the throughput of the download was not measured
3. a timer was used, instead of a histogram, with timers emitting much more unneeded metrics compared to a histogram. Across a huge fleet of agents, this makes a big difference, so I am now using a histogram for efficiency.

Note: we also emit metainfo download latency server-side in tracker, but we cannot rely on that metric, as the tracker has an nginx cache in front of the application code, therefore emitting metrics only on cache misses, which provides a skewed distribution.

[sambhav-jain-16]

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