Add warning for incompatible dogstastd-ruby version

[marcotc]

This PR adds a one-time warning when ddtrace tries to instantiate Datadog::Statsd with a version that can cause unbounded memory usage by leaking threads #1491.

We've already updated our documentation instructing new users to not install dogstastd-ruby >= 5.0: #1533. Versions < 5.0 are confirmed safe.

There's only one place in ddtrace where Datadog::Statsd instances are created, so the warning was placed at that point.

[codecov-commenter]

Codecov Report

Merging #1544 (75113a0) into master (75bec75) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1544   +/-   ##
=======================================
  Coverage   98.23%   98.24%           
=======================================
  Files         878      878           
  Lines       42337    42361   +24     
=======================================
+ Hits        41591    41616   +25     
+ Misses        746      745    -1     

Impacted Files	Coverage Δ
lib/ddtrace/metrics.rb	96.87% <100.00%> (+0.23%)	⬆️
spec/ddtrace/metrics_spec.rb	100.00% <100.00%> (ø)
spec/ddtrace/profiling/ext/forking_spec.rb	100.00% <0.00%> (+0.61%)	⬆️

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