add Elixir modules for baggage and context

[tsloughter]

Resolves #120

[codecov]

Codecov Report

Merging #121 into master will increase coverage by 0.10%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   32.01%   32.12%   +0.10%     
==========================================
  Files          61       63       +2     
  Lines        2927     2945      +18     
==========================================
+ Hits          937      946       +9     
- Misses       1990     1999       +9     

Flag	Coverage Δ
#api	50.00% <68.42%> (+0.28%)	⬆️
#elixir	16.84% <68.42%> (+3.99%)	⬆️
#erlang	31.85% <25.00%> (-0.18%)	⬇️
#exporter	17.05% <ø> (ø)
#sdk	63.86% <ø> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
apps/opentelemetry_api/src/otel_ctx.erl	83.33% <ø> (-8.34%)	⬇️
...ps/opentelemetry_api/lib/open_telemetry/baggage.ex	60.00% <60.00%> (ø)
apps/opentelemetry_api/lib/open_telemetry/ctx.ex	70.00% <70.00%> (ø)
apps/opentelemetry_api/src/otel_baggage.erl	81.48% <75.00%> (-1.86%)	⬇️
apps/opentelemetry/src/otel_batch_processor.erl	72.41% <0.00%> (-1.15%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b002ce...919d552. Read the comment docs.

[hauleth]

Out of interest. Why do we implement such modules that are fully defdelegate. I would understand situation when we need to have some changes to be made to make it usable with Elixir, like defining Elixir macros or changing order of arguments. `in this case I feel that it is a little bit unneeded in most cases, as calling Erlang functions is pretty straightforward.

[tsloughter]

Because modules like Tracer and the metrics modules exist that do do more than just defdelegate. It was discussed at some point that it is better than just having some modules with Elixir counterpart to have all.

The SDK has no Elixir modules because they would all just be defdelegates, but the API feels more consistent if you aren't mixing different types of module calls.