PR25448: debuginfod: add transfer performance metrics

PR25448: debuginfod: add transfer performance metrics

We now export metrics related to the time taken and data sent,
from which prometheus type tools can compute nice time series
with averages.

http_responses_duration_milliseconds_count{code="200"} 63
http_responses_duration_milliseconds_count{code="404"} 2
http_responses_duration_milliseconds_count{code="503"} 1
http_responses_duration_milliseconds_sum{code="200"} 66
http_responses_duration_milliseconds_sum{code="404"} 2
http_responses_duration_milliseconds_sum{code="503"} 0
http_responses_transfer_bytes_count{code="200"} 63
http_responses_transfer_bytes_count{code="404"} 2
http_responses_transfer_bytes_count{code="503"} 1
http_responses_transfer_bytes_sum{code="200"} 425177
http_responses_transfer_bytes_sum{code="404"} 18
http_responses_transfer_bytes_sum{code="503"} 37

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>