Add attribute linting and ecosystem support.

[garthk]

I keep repeating the same Elixir code in my reporters, middleware, and unit tests. This commit extracts all that code, cleans it up, and makes it available for everyone else working with Opencensus in Elixir.

• Alter Opencensus.Trace.with_child_span/2 attribute handling
• Add Opencensus.Trace.put_span_attributes/1
• Add Opencensus.Trace.async/1
• Add Opencensus.Trace.async/3
• Add Opencensus.Trace.await/1
• Add Opencensus.Span struct derived from the record
• Add Opencensus.Span.from/1
• Add Opencensus.Span.load/1
• Add Opencensus.SpanContext struct derived from the record
• Add Opencensus.SpanContext.from/1
• Add Opencensus.SpanContext.hex_trace_id/1
• Add Opencensus.SpanContext.hex_span_id/1
• Add Opencensus.TestSupport.SpanCaptureReporter
• Add Opencensus.Attributes.process_attributes/1
• Add Opencensus.Attributes.process_attributes/2
• Add Opencensus.Attributes.default_attributes/1

The new attribute handling:

• Drops span attributes with unsupported values

• Flattens nested maps

• Documents the value support limits imposed by the Opencensus wire protocol and :ocp.put_span_attribute/2

• Preserves the :module, :file, :function, :line, and :default magic attributes

Misc:

• mix credo
• mix test.watch

[garthk]

Called out the most controversial parts in bold. Strikes me it's irresponsible to let package developers set span attributes that'll result in their spans getting dumped. Even if we decide that's the job of the reporters, we'll still want to offer the code to get the heavy lifting right. The rest is just useful.

[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 2.62%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   95.45%   98.07%   +2.62%     
==========================================
  Files           2        6       +4     
  Lines          22       52      +30     
==========================================
+ Hits           21       51      +30     
  Misses          1        1

Impacted Files	Coverage Δ
lib/opencensus/span.ex	100% <100%> (ø)
lib/opencensus/span_context.ex	100% <100%> (ø)
lib/opencensus/logger.ex	100% <100%> (ø)	⬆️
lib/opencensus/attributes.ex	100% <100%> (ø)
...b/opencensus/test_support/span_capture_reporter.ex	100% <100%> (ø)
lib/opencensus/trace.ex	88.88% <88.88%> (-5.56%)	⬇️

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[tsloughter]

Can this also include @zachdaniel 's idea to have default attributes be a config option? Instead of all this confusion with a list of default attributes like [:line, :file, ...] just include them in the attributes if some opencensus_elixir configuration is set to true.

That or just always include them as we can say "don't use the Elixir macro" if you don't want the extra attributes.

[garthk]

I made all four compulsory in my internal macro. Checking app config and including them or excluding them by default is a decent enough middle ground.

[garthk]

Split #20 out; will come back with another PR for attribute control.