metrics: Mitigate breakage in structopt-derive v0.3.6

[timotree3]

PR summary

Until either this PR is merged or TeXitoi/structopt#315 is fixed, all new CI runs will fail.

This PR

• Mitigates the breakage introduced in structopt-derive v0.3.6 by preventing holochain_metrics from using that version of structopt-derive.
  • The way we prevent this is by adding an unused dependency of structopt-derive = "=0.3.3" to our Cargo.toml.

testing/benchmarking notes

N/A

followups

N/A

changelog

• if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• this code has been documented according to our docs checklist

[zippy]

Woot! Fabulous catch. Thank you @timotree3