Remove from_env and use env vars by default

[jtescher]

Currently environment variables are not picked up by default and must be opted in via explicit calls to from_env in builders. This change updates this behavior by removing the from_env methods and instead checking the respective variables in constructors.

Resolves #434
Resolves #268
Resolves #168

[codecov]

Codecov Report

Merging #459 (892a98a) into main (f79586b) will increase coverage by 0.69%.
The diff coverage is 75.40%.

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   47.88%   48.57%   +0.69%     
==========================================
  Files          95       95              
  Lines        8736     8747      +11     
==========================================
+ Hits         4183     4249      +66     
+ Misses       4553     4498      -55     

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/lib.rs	89.36% <ø> (ø)
opentelemetry/src/sdk/trace/id_generator/aws.rs	76.92% <ø> (ø)
opentelemetry/src/sdk/trace/provider.rs	58.18% <0.00%> (ø)
opentelemetry/src/sdk/trace/config.rs	36.95% <40.00%> (+7.22%)	⬆️
opentelemetry/src/sdk/trace/span_processor.rs	78.16% <80.00%> (-2.11%)	⬇️
opentelemetry/src/sdk/resource.rs	79.31% <92.30%> (+1.74%)	⬆️
opentelemetry-jaeger/src/exporter/mod.rs	40.89% <100.00%> (+0.57%)	⬆️
...ntelemetry/src/sdk/metrics/aggregators/ddsketch.rs	76.95% <0.00%> (+0.19%)	⬆️
... and 2 more

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[TommyCpp]

LGTM 👍