metrics feature flag

[finiteprods]

This merge request updates the manifest to support an "opt-in" for a forthcoming feature metrics. Making this feature optional means we can avoid the overhead (both at run time and compile time) associated with this feature when we don't need it on.

There is also a small test app bin/hello-metrics.rs to illustrate how to use it in our library code, which will be removed in the final PR.
By default, without the feature:

$ cargo run --bin hello-metrics
Hello World!

With the feature:

$ cargo run --features "metrics" --bin hello-metrics
Hello World!
Influx says Hi!

[codecov]

Codecov Report

Merging #489 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
- Coverage   51.56%   51.49%   -0.07%     
==========================================
  Files          63       63              
  Lines        3105     3105              
==========================================
- Hits         1601     1599       -2     
- Misses       1504     1506       +2     

Impacted Files	Coverage Δ
rust/src/state_machine/phases/update.rs	66.66% <0.00%> (-1.91%)	⬇️

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[Robert-Steiner]

Looks good👍 I think we can already merge it so that you can use it in the metrics settings task