rust: add `--verbose` to control log level (#4398)
rust: add `--verbose` to control log level (#4398)

Summary:
We now only show WARN logs by default, showing INFO at `-v` or all logs
at `-vv` or higher.

Test Plan:
Run `bazel run //tensorboard/data/server` with a `--logdir` that has
graphs. By default, that should print a warning that graphs are not yet
supported. With `-v` or `--verbose` or `--verbose=1`, it should also
print load cycle timing information. With `-vv` or `--verbose --verbose`
or `--verbose=2` (or more), it should print a debug log including the
parsed options, and also some `mio::poll` internals.

wchargin-branch: rust-verbose-flag

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard