Ensure special tags are always strings

[marcotc]

Some Span tags have special handling in the Datadog Agent. Here's the list of these tags.
These tags have to always be reported as string Tags, not numerical Metrics.

One example of issues caused by setting these values as numerical is this:

span.set_tag('version', 123)

In this case, version will not correctly set the span version, thus not correctly updating the Datadog backend version value.

This PR adds a check and ensures that these values are always string tags.

[codecov-commenter]

Codecov Report

Merging #1556 (4a1818d) into master (50e2f8e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1556   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         878      878           
  Lines       42390    42407   +17     
=======================================
+ Hits        41643    41660   +17     
  Misses        747      747           

Impacted Files	Coverage Δ
lib/ddtrace/configuration/components.rb	98.19% <100.00%> (ø)
lib/ddtrace/span.rb	97.00% <100.00%> (+0.07%)	⬆️
spec/ddtrace/span_spec.rb	100.00% <100.00%> (ø)

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