Use Array#to_h instead of Hash[array]

[marcotc]

This PR uses Array#to_h, instead of Hash[array], as this method is available since Ruby 2.1.

Array#to_h has cleaner syntax, is much faster with empty arrays, and is slightly faster with populated arrays:

require 'benchmark/ips'

arr = []
a16 = 16.times.map{|i|[i.to_s.to_sym, 'test']}

Benchmark.ips do |x|
  x.config(time: 2, warmup: 0.1) if x.respond_to? :config

  x.report('empty array: Hash[A]') { Hash[arr] }
  x.report('empty array: A.to_h ') { arr.to_h }
  x.report('16 elements: Hash[A]') { Hash[a16] }
  x.report('16 elements: A.to_h ') { a16.to_h }

  x.compare!
end

# empty array: A.to_h : 13333116.0 i/s
# empty array: Hash[A]:  8432958.2 i/s - 1.58x  (± 0.00) slower
# 16 elements: A.to_h :   770205.8 i/s - 17.31x  (± 0.00) slower
# 16 elements: Hash[A]:   697552.1 i/s - 19.11x  (± 0.00) slower

Ruby 2.0 deprecation checklist

• Remove 2.0 from CI
• Remove 2.0-specific code paths
• Update to 2.1+ standards: https://github.com/ruby/ruby/blob/v2_1_0/NEWS
  • Module#include and Module#prepend are now public methods.
  • Exception#cause (no changes)
  • Array#to_h converts an array of key-value pairs into a Hash.
  • Kernel#singleton_method
  • Process.clock_gettime
  • Enumerable#to_h converts a list of key-value pairs into a Hash.
  • Now the default values of keyword arguments can be omitted. Those “required keyword arguments” need giving explicitly at the call time.
• Update documentation

[codecov-commenter]

Codecov Report

Merging #1523 (6e3fd4f) into master (2dcc26b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1523   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files         862      862           
  Lines       41720    41721    +1     
=======================================
+ Hits        40978    40979    +1     
  Misses        742      742           

Impacted Files	Coverage Δ
lib/ddtrace/configuration/base.rb	97.61% <ø> (-0.06%)	⬇️
lib/ddtrace/configuration/settings.rb	100.00% <100.00%> (ø)
lib/ddtrace/diagnostics/environment_logger.rb	98.05% <100.00%> (+0.01%)	⬆️
lib/ddtrace/runtime/metrics.rb	96.36% <100.00%> (+0.06%)	⬆️
lib/ddtrace/tracer.rb	94.64% <100.00%> (ø)

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[ivoanjo]

LGTM!

You probably also want to re-enable Style/HashConversion in .rubocop.yml.