[receiver/haproxy] fix reading more than 4096 bytes

[atoulme]

Description:
This makes sure to read the whole data when interacting over the socket with haproxy.

Link to tracking Issue:
Fixes #32652

Testing:
Tested manually.

[atoulme]

tested manually with the following:
haproxy.cfg:

global
  stats socket /tmp/admin.sock mode 660

frontend gr-prod-rw
    bind 0.0.0.0:3307     
    mode tcp
    option contstats
    option dontlognull
    option clitcpka
    default_backend servers

backend servers
    server server1 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server2 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server3 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server4 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server5 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server6 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server7 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server8 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server9 10.0.68.39:3307 check inter 10000 rise 1 fall 2 weight 50
    server server10 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server11 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server12 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server13 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server14 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server15 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server16 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server17 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server18 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server19 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup
    server server20 10.0.68.74:3307 check inter 10000 rise 1 fall 2 weight 50 backup
    server server21 10.0.68.20:3307 check inter 10000 rise 1 fall 2 weight 1 backup

otel.yaml

receivers:
  haproxy:
    endpoint: /tmp/admin.sock
    collection_interval: 10s

exporters:
  debug:
    verbosity: detailed

service:
  pipelines:
    metrics:
      receivers: [haproxy]
      exporters: [debug]

Verified that the output of the command is more than 4096 bytes:

echo 'show stat' | socat /tmp/admin.sock stdio | wc -c
    7091

Ran the collector and saw metrics come in.