update tracing-opentelemetry to 0.16

[Geal]

fix #17

this update requires moving to tracing-subscriber 0.3. It removes chrono
from transitive dependencies, following
https://rustsec.org/advisories/RUSTSEC-2020-0159.html

Context:

• tracing-opentelemetry 0.16 release: opentelemetry: prepare to release v0.16.0 tokio-rs/tracing#1682
• tracing subscriber 0.3 release with the removal of chrono: subscriber: prepare to release v0.3.0 tokio-rs/tracing#1677

[Geal]

we would like to upgrade tracing-subscriber in https://github.com/apollographql/router, and reqwest-tracing is the last component we need to update for that

[LukeMathWalker]

Thanks for the PR!
This was released as reqwest-tracing 0.2.0.

[Geal]

Awesome, thanks!