refactor: Default to Proxy Tracer in instrumentations

While working on #677,
I ran into an issue where the base instrumentation tracer was not being automatically upgraded
after the SDK initialization was complete.

I am not sure why this was implemented this way but I figured other instrumentations would need a reference to a proxy tracer in the future.

open-telemetry/opentelemetry-ruby#671

instrumentation/base/lib/opentelemetry/instrumentation/base.rb

@@ -204,7 +204,7 @@ def initialize(name, version, install_blk, present_blk,
         @config = {}
         @installed = false
         @options = options
-        @tracer = OpenTelemetry::Trace::Tracer.new
+        @tracer = OpenTelemetry.tracer_provider.tracer(name, version)
       end
       # rubocop:enable Metrics/ParameterLists
 
@@ -220,7 +220,6 @@ def install(config = {})
         return false unless installable?(config)
 
         instance_exec(@config, &@install_blk)
-        @tracer = OpenTelemetry.tracer_provider.tracer(name, version)
         @installed = true
       end