Log human-readable content length (bytesize).

[danielballan]

To do this right, we need #116 and #281. This is on ice until those are addressed.

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When the Content-Length header (giving the bytesize of a response) is large, it is hard to read. This puts the bitesize in human readable units using a metric prefix at the front of each response log message.

A small response's log message looks like

10:26:47.582 <- 200 514.0B server:nginx/1.18.0 (Ubuntu) date:Thu, 04 Aug 2022 14:26:47 GMT content-type:application/x-msgpack content-length:514 connection:keep-alive etag:073e89cae536cc46212d597e25210c72 server-timing:acl;dur=0.0, tok;dur=0.0, pack;dur=0.0, app;dur=6.4

and a large response's log message looks like:

10:27:38.753 <- 200 703.1MB server:nginx/1.18.0 (Ubuntu) date:Thu, 04 Aug 2022 14:27:38 GMT content-type:application/octet-stream content-length:720000 connection:keep-alive etag:bcdc0ac54467c43b37038182b9f0e0fb server-timing:acl;dur=0.0, read;dur=0.7, tok;dur=0.5, pack;dur=0.5, app;dur=6.9