Explicitly require Client class (#87)

Explicitly require Client class (#87)

Many of the instrumentation implementations reference PrometheusExporter::Client without explicitly requiring that class first. Adding a require here means they can be included as one file reference by consumers.

diff --git a/lib/prometheus_exporter/instrumentation.rb b/lib/prometheus_exporter/instrumentation.rb
index d160615..1e3a384 100644
--- a/lib/prometheus_exporter/instrumentation.rb
+++ b/lib/prometheus_exporter/instrumentation.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require_relative "client"
 require_relative "instrumentation/process"
 require_relative "instrumentation/method_profiler"
 require_relative "instrumentation/sidekiq"

GitHub sha: 3614e77f