Implement timeout for force_flush and shutdown.

[TommyCpp]

• Add the envionment variable support for export timeout.
• Add support of time out when exporting.
• Added TraceError enum, still need to add other error type.

Note that the spec only has configuration for maximum time to export data. So the timeout is based on the export time.

Also included the update from otel-proto

[codecov]

Codecov Report

Merging #362 (1cc920d) into master (310df35) will increase coverage by 0.64%.
The diff coverage is 80.89%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   54.56%   55.20%   +0.64%     
==========================================
  Files          70       71       +1     
  Lines        5914     6048     +134     
==========================================
+ Hits         3227     3339     +112     
- Misses       2687     2709      +22     

Impacted Files	Coverage Δ
opentelemetry/src/api/trace/mod.rs	0.00% <0.00%> (ø)
opentelemetry/src/exporter/trace/mod.rs	64.19% <0.00%> (-3.34%)	⬇️
opentelemetry/src/sdk/trace/span_processor.rs	83.15% <84.56%> (+2.10%)	⬆️
opentelemetry/src/sdk/trace/provider.rs	75.51% <100.00%> (+0.51%)	⬆️
opentelemetry/src/testing/trace.rs	75.75% <0.00%> (+4.54%)	⬆️

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[frigus02]

This looks like the correct approach to me. I didn't manage to review the tests, yet. Will try to do this later today.

[frigus02]

There is a clippy warning:

casting integer literal to usize is unnecessary

Otherwise this looks good to me 👍

[jtescher]

Looks like cargo fmt check is failing still

[jtescher]

Looks good thanks @TommyCpp, if there are changes we want to do for #364 we can do them in a subsequent PR