Merge branch 'marc/271-client-support-messagepack-for-process-bandwidth-monitoring' into 'master'

Resolve "Client: Support MessagePack for process bandwidth monitoring"

GekkoFS clients support capturing the I/O traces of each individual process and periodically exporting them to a given
file or ZeroMQ sink via the TCP protocol.
To use this feature, the corresponding ZeroMQ (`libzmq` and `cppzmq`) dependencies are required which can be found in
the `default_zmq` dependency profile.
In addition, GekkoFS must be compiled with client metrics enabled (disabled by default) via the CMake argument
`-DGKFS_ENABLE_CLIENT_METRICS=ON`.

Client metrics are individually enabled per GekkoFS client process via the following environment variables:

- `LIBGKFS_ENABLE_METRICS=ON` enables capturing client-side metrics.
- `LIBGKFS_METRICS_FLUSH_INTERVAL=10` sets the flush interval to 10 seconds (defaults to 5). All outstanding client
  metrics are flushed when the process ends.
- `LIBGKFS_METRICS_PATH=<path>` sets the path to flush client-metrics (defaults to `/tmp/gkfs_client_metrics`).
- `LIBGKFS_METRICS_IP_PORT=127.0.0.1:5555` enables flushing to a set ZeroMQ server. This option disables flushing to a
  file.

The ZeroMQ export can be tested via the `gkfs_clientmetrics2json` application which is built when enabling the CMake
option `-DGKFS_BUILD_TOOLS=ON`:

- Starting the ZeroMQ server: `gkfs_clientmetrics2json tcp://127.0.0.1:5555`
- `gkfs_clientmetrics2json <path>` can also be used to unpack the Messagepack export from a file.
  Examplarily output with the ZeroMQ sink enabled when running:
  `LD_PRELOAD=libgkfs_intercept.so LIBGKFS_ENABLE_METRICS=ON LIBGKFS_METRICS_IP_PORT=127.0.0.1:5555 gkfs cp testfile /tmp/gkfs_mountdir/testfile`:

```bash
~ $ gkfs_clientmetrics2json tcp://127.0.0.1:5555
Binding to: tcp://127.0.0.1:5555
Waiting for message...

Received message with size 68
Generated JSON:
[extra]avg_thruput_mib: [221.93,175.87,266.81,135.69]
end_t_micro: [8008,12396,16006,18454]
flush_t: 18564
hostname: "evie"
io_type: "w"
pid: 1259304
req_size: [524288,524288,524288,229502]
start_t_micro: [5755,9553,14132,16841]
total_bytes: 1802366
total_iops: 4
```

Closes #271

Closes #271

See merge request !176