Resolve warnings

[abicky]

This PR resolves the following warnings:

• instance variable @variable_name not initialized
• ambiguous first argument; put parentheses or a space even after `/' operator
• Using expect { }.not_to raise_error(SpecificErrorClass, message) risks false positives

Before

% ruby --version
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]
% bundle exec rake >/dev/null 2>&1 | grep -i warning: | sed -E 's/^[.*]+//g' | sort -u
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/lib/datadog/statsd/forwarder.rb:67: warning: instance variable @telemetry not initialized
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/lib/datadog/statsd/sender.rb:105: warning: instance variable @flush_timer not initialized
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/lib/datadog/statsd/sender.rb:97: warning: instance variable @flush_timer not initialized
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/lib/datadog/statsd/timer.rb:44: warning: instance variable @thread not initialized
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/spec/statsd/version_spec.rb:6: warning: ambiguous first argument; put parentheses or a space even after `/' operator
WARNING: Using `expect { }.not_to raise_error(SpecificErrorClass, message)` risks false positives, since literally any other error would cause the expectation to pass, including those raised by Ruby (e.g. `NoMethodError`, `NameError` and `ArgumentError`), meaning the code you are intending to test may not even get reached. Instead consider using `expect { }.not_to raise_error` or `expect { }.to raise_error(DifferentSpecificErrorClass)`. This message can be suppressed by setting: `RSpec::Expectations.configuration.on_potential_false_positives = :nothing`. Called from /Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/spec/statsd_spec.rb:1015:in `block (5 levels) in <top (required)>'.
%

% ruby --version
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin20]
% bundle exec rake >/dev/null 2>&1 | grep -i warning: | sed -E 's/^[.*]+//g' | sort -u
/Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/spec/statsd/version_spec.rb:6: warning: ambiguity between regexp and two divisions: wrap regexp in parentheses or add a space after `/' operator
WARNING: Using `expect { }.not_to raise_error(SpecificErrorClass, message)` risks false positives, since literally any other error would cause the expectation to pass, including those raised by Ruby (e.g. `NoMethodError`, `NameError` and `ArgumentError`), meaning the code you are intending to test may not even get reached. Instead consider using `expect { }.not_to raise_error` or `expect { }.to raise_error(DifferentSpecificErrorClass)`. This message can be suppressed by setting: `RSpec::Expectations.configuration.on_potential_false_positives = :nothing`. Called from /Users/arabiki/ghq/src/github.com/DataDog/dogstatsd-ruby/spec/statsd_spec.rb:1015:in `block (5 levels) in <top (required)>'.
%

After

% ruby --version
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]
% bundle exec rake >/dev/null 2>&1 | grep -i warning: | sed -E 's/^[.*]+//g' | sort -u
%

% ruby --version
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin20]
% bundle exec rake >/dev/null 2>&1 | grep -i warning: | sed -E 's/^[.*]+//g' | sort -u
%

[djmitche]

These changes look great -- thanks!