http_client: fix muti part http2 request not working

[flobz]

When the request body is quite big, it's split but only the first part is send multiple time instead of sending all parts.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
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Backporting

• Backport to latest stable release.

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[flobz]

Fluent config:

[SERVICE]
    Flush     1
    Daemon    off
    Log_Level debug


[INPUT]
    Name                 node_exporter_metrics
    Tag                  system_metrics
    Scrape_interval      10

[OUTPUT]
    Name                 opentelemetry
    Match                *
    Host                 127.0.0.1
    Port                 4317
    grpc                 true
    http2                force
    Log_response_payload True
    Tls                  Off

Open telemetry collector config:

receivers:
  otlp:
    protocols:
     grpc:
        endpoint: 0.0.0.0:4317
       
exporters:
      debug:
        verbosity: detailed
service:
  pipelines:
    metrics:
        receivers: [otlp]
        exporters: [debug]
  telemetry:
    logs:
     level: debug
     development: true