msrv: bump from 1.49 to 1.55

[TommyCpp]

• Follow up Allow O(1) get operations for SpanBuilder::attributes [breaking] #799. Update the MSRV in README files.
• Pin the async-std version to 1.8 as any version > 1.8 will requires rustc 1.59 support, which breaks our MSRV guarantee. Ideally we should revert this change once Restore num_cpus and reduce MSRV? async-rs/async-global-executor#10 resolves and the new version releases
• Revert the MSRV in ci config from 1.59 to 1.55, it was previously bumped to unblock the PR merging.

[codecov]

Codecov Report

Merging #811 (cff2427) into main (3806258) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #811   +/-   ##
=====================================
  Coverage   69.8%   69.8%           
=====================================
  Files        110     110           
  Lines       9072    9072           
=====================================
+ Hits        6334    6337    +3     
+ Misses      2738    2735    -3     

Impacted Files	Coverage Δ
opentelemetry-api/src/lib.rs	100.0% <ø> (ø)
opentelemetry-jaeger/src/lib.rs	93.0% <ø> (ø)
opentelemetry-zipkin/src/lib.rs	100.0% <ø> (ø)
opentelemetry/src/lib.rs	100.0% <ø> (ø)
opentelemetry-http/src/lib.rs	7.0% <0.0%> (-5.3%)	⬇️
opentelemetry-jaeger/src/exporter/mod.rs	54.0% <0.0%> (+3.0%)	⬆️

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[jtescher]

We may still want to support 1.49 as many people integrate via tracing https://github.com/tokio-rs/tracing/tree/master/tracing#supported-rust-versions

[jtescher]

Which changes would we need to revert to continue supporting 1.49?

[TommyCpp]

Which changes would we need to revert to continue supporting 1.49?

The bump was introduced by #799. I think alternatively we can rewrite part of the indexmap to avoid the bump.

Although Tokio ecosystem promise 6-month compatibility. 1.55 was released in Sep 2021 so it's still consistent with that promise.

Any thoughts?

[jtescher]

Nvm we could open an issue to bump tracing as well, but probably simpler to retain 1.49 support if we can't get that in.

[TommyCpp]

but probably simpler to retain 1.49 support if we can't get that in.

I will give it a try! I will merge this for now so that we can maintain at least 1.55 MSRV when building the new PRs. The current one in main branch breaks our MSRV promise.