Add optional bitrate HTTP parameter

[v-zhuravlev]

This allows to additionally set --bitrate iperf3 flag that can limit bitrate used for testing. This can be particularly helpful for testing channels for minimally accepted real bandwidth on regular basis to avoid real users interference:

iperf3 --help
...
-b, --bitrate #[KMG][/#] target bitrate in bits/sec (0 for unlimited)
(default 1 Mbit/sec for UDP, unlimited for TCP)
(optional slash and packet count for burst mode)
...

It can be set like this:

    - name: iperf3_exporter
      scrape_configs:
        - job_name: 'iperf3'
          metrics_path: /probe
          scrape_interval: 15m
          static_configs:
            - targets:
                 - xyz.example
          params:
            bitrate: 10M
            port: ['5201']
          relabel_configs:
            - source_labels: [__address__]
              target_label: __param_target
            - source_labels: [__param_target]
              target_label: instance
            - target_label: __address__
              replacement: "localhost:9579"