Implement Display trait for Key and Value types

[djc]

This has the advantage that you can write these types into a string buffer without allocating. The split between &Value and Value From impls was not used efficiently in some of the other crates, and that is fixed here as well.

In addition, I believe this makes the API more idiomatic.

[codecov]

Codecov Report

Merging #353 (353972c) into master (5cf034c) will decrease coverage by 0.19%.
The diff coverage is 37.93%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
- Coverage   54.21%   54.02%   -0.20%     
==========================================
  Files          69       70       +1     
  Lines        5829     5833       +4     
==========================================
- Hits         3160     3151       -9     
- Misses       2669     2682      +13     

Impacted Files	Coverage Δ
opentelemetry/src/api/labels/encoder.rs	0.00% <0.00%> (ø)
opentelemetry/src/api/core.rs	55.72% <44.44%> (-1.31%)	⬇️
opentelemetry/src/api/baggage.rs	80.95% <66.66%> (ø)
opentelemetry/src/sdk/propagation/baggage.rs	96.42% <100.00%> (ø)
opentelemetry/src/exporter/trace/mod.rs	67.53% <0.00%> (-2.74%)	⬇️
opentelemetry/src/api/trace/tracer.rs	37.83% <0.00%> (-1.64%)	⬇️
opentelemetry/src/api/trace/noop.rs	68.26% <0.00%> (-0.67%)	⬇️
opentelemetry/src/testing/trace.rs	72.34% <0.00%> (-0.58%)	⬇️
...ntelemetry/src/sdk/metrics/aggregators/ddsketch.rs	78.28% <0.00%> (-0.32%)	⬇️
opentelemetry/src/sdk/trace/tracer.rs	74.50% <0.00%> (-0.17%)	⬇️
... and 3 more

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[jtescher]

Looks good, some new nightly clippy lints, but they can be addressed in subsequent PRs

[djc]

Oh, I didn't see those. None of them seem to be related to the changes here, though.