Add probe_http_uncompressed_body_length metric

[strangeman]

Partial solution for #337

I think this is a suitable solution for cases "we need to monitor the size of very important page and alert when it dramatically changed". I just get the size of the response body (which transparently decompressed by http.Response) and expose it.

Page with disabled compression:

http://localhost:9115/probe?target=example.com&module=http_2xx

# HELP probe_http_content_length Length of http content response
# TYPE probe_http_content_length gauge
probe_http_content_length 1270
# HELP probe_http_uncompressed_body_length Length of uncompressed response body
# TYPE probe_http_uncompressed_body_length gauge
probe_http_uncompressed_body_length 1270

Page with enabled compression:

http://localhost:9115/probe?target=github.com&module=http_2xx

# HELP probe_http_content_length Length of http content response
# TYPE probe_http_content_length gauge
probe_http_content_length -1

# HELP probe_http_uncompressed_body_length Length of uncompressed response body
# TYPE probe_http_uncompressed_body_length gauge
probe_http_uncompressed_body_length 134683

Nonexistent site:

http://localhost:9115/probe?target=notexistent.site&module=http_2xx

# HELP probe_http_content_length Length of http content response
# TYPE probe_http_content_length gauge
probe_http_content_length 0

# HELP probe_http_uncompressed_body_length Length of uncompressed response body
# TYPE probe_http_uncompressed_body_length gauge
probe_http_uncompressed_body_length 0

# HELP probe_success Displays whether or not the probe was a success
# TYPE probe_success gauge
probe_success 0

[brian-brazil]

Why not keep using ioutil.Discard?

[strangeman]

You're right. I misunderstood the documentation.

[brian-brazil]

Thanks!