Add the open_fds metric to the docs (#4220)

This change adds the missing `open_fds` metric to the
`tenzir.metrics.process` documentation.

web/docs/operators/metrics.md

@@ -101,6 +101,7 @@ Contains a measurement of the amount of memory used by the `tenzir-node` process
 |`current_memory_usage`|`uint64`|The memory currently used by this process.|
 |`peak_memory_usage`|`uint64`|The peak amount of memory, in bytes.|
 |`swap_space_usage`|`uint64`|The amount of swap space, in bytes. Only available on Linux systems.|
+|`open_fds`|`uint64`|The amount of open file descriptors by the node. Only available on Linux systems.|
 
 ## Examples