What is blackbox_exporter probe_http_uncompressed_body_length exactly?

What is the metric probe_http_uncompressed_body_length exactly? How to reproduce this metric with curl? Why is it 0 at me? Not all my targets are 0, but many targets that return a small JSON document show with 0.

Debug output:

# HELP probe_dns_lookup_time_seconds Returns the time taken for probe dns lookup in seconds
# TYPE probe_dns_lookup_time_seconds gauge
probe_dns_lookup_time_seconds 0.000561499
# HELP probe_duration_seconds Returns how long the probe took to complete in seconds
# TYPE probe_duration_seconds gauge
probe_duration_seconds 0.007946717
# HELP probe_failed_due_to_regex Indicates if probe failed due to regex
# TYPE probe_failed_due_to_regex gauge
probe_failed_due_to_regex 0
# HELP probe_http_content_length Length of http content response
# TYPE probe_http_content_length gauge
probe_http_content_length 195
# HELP probe_http_duration_seconds Duration of http request by phase, summed over all redirects
# TYPE probe_http_duration_seconds gauge
probe_http_duration_seconds{phase="connect"} 0.003066714
probe_http_duration_seconds{phase="processing"} 0.003410863
probe_http_duration_seconds{phase="resolve"} 0.000561499
probe_http_duration_seconds{phase="tls"} 0
probe_http_duration_seconds{phase="transfer"} 0.00027692
# HELP probe_http_redirects The number of redirects
# TYPE probe_http_redirects gauge
probe_http_redirects 0
# HELP probe_http_ssl Indicates if SSL was used for the final redirect
# TYPE probe_http_ssl gauge
probe_http_ssl 0
# HELP probe_http_status_code Response HTTP status code
# TYPE probe_http_status_code gauge
probe_http_status_code 200
# 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_http_version Returns the version of HTTP of the probe response
# TYPE probe_http_version gauge
probe_http_version 1.1
# HELP probe_ip_addr_hash Specifies the hash of IP address. It's useful to detect if the IP address changes.
# TYPE probe_ip_addr_hash gauge
probe_ip_addr_hash xxxx
# HELP probe_ip_protocol Specifies whether probe ip protocol is IP4 or IP6
# TYPE probe_ip_protocol gauge
probe_ip_protocol 4
# HELP probe_success Displays whether or not the probe was a success
# TYPE probe_success gauge
probe_success 1

config of module

Module configuration:
prober: http
timeout: 30s
http:
    preferred_ip_protocol: ip4
    ip_protocol_fallback: true
    method: GET
    fail_if_body_not_matches_regexp:
        - '"name": "schema.version"'
    basic_auth:
        username: <user>
        password: <secret>
    tls_config:
        insecure_skip_verify: true
tcp:
    ip_protocol_fallback: true
icmp:
    ip_protocol_fallback: true
dns:
    ip_protocol_fallback: true

Same request on commandline:

curl -s --user $USER@$CUSTOMER:$PASSWORD http://example.org/controller/rest/configuration?output=JSON&name=schema.version&pretty=true
[{
  "updateable": false,
  "scope": "cluster",
  "name": "schema.version",
  "description": "Current version of the controller's schema, used to track upgrades.",
  "value": "020-007-002-000"
}]