feat: add "List Service Instances for Service" call (#8)

This should be the minimum required for querying the list of healthy &
unhealthy instances for a given service.

lib/consulkit/client.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'consulkit/configurable'
+require 'consulkit/client/health'
 require 'consulkit/client/kv'
 require 'consulkit/client/session'
 
@@ -9,6 +10,7 @@ module Consulkit
   class Client
 
     include Consulkit::Configurable
+    include Consulkit::Client::Health
     include Consulkit::Client::KV
     include Consulkit::Client::Session
 

lib/consulkit/client/health.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module Consulkit
+  class Client
+    # Methods for querying health checks registered with Consul.
+    module Health
+
+      # Returns the list of service instances providing the given service, including health check
+      # information.
+      #
+      # @see https://developer.hashicorp.com/consul/api-docs/health#list-service-instances-for-service
+      #
+      # @param key [String] the key to read.
+      # @option query_params [Hash] optional query parameters.
+      # @option query_params [Boolean] :passing
+      # @option query_params [String] :filter
+      #
+      # @yield [Faraday::Response] The response from the underlying Faraday library.
+      #
+      # @return [Array<Hash>]
+      def health_list_service_instances(service, query_params = {})
+        response = get("/v1/health/service/#{service}", query_params)
+
+        if block_given?
+          yield response
+        else
+          response.body
+        end
+      end
+
+    end
+  end
+end

spec/consulkit/client/health_spec.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+describe Consulkit::Client::Health, :vcr do
+  before do
+    @client  = Consulkit.client
+    @service = 'consul'
+  end
+
+  describe 'health_list_service_instances' do
+    it 'lists the instances' do
+      instances = @client.health_list_service_instances(@service)
+
+      expect(instances.length).to be 1
+
+      first_instance = instances.first
+
+      expect(first_instance.dig('Node', 'Node')).to eq('localhost')
+      expect(first_instance.dig('Service', 'ID')).to eq('consul')
+      expect(first_instance.dig('Checks', 0, 'CheckID')).to eq('serfHealth')
+    end
+
+    it 'passes query params' do
+      instances = @client.health_list_service_instances(@service, filter: 'Checks.Status == "passing"')
+
+      expect(instances.length).to be 1
+
+      first_instance = instances.first
+
+      expect(first_instance.dig('Node', 'Node')).to eq('localhost')
+      expect(first_instance.dig('Service', 'ID')).to eq('consul')
+      expect(first_instance.dig('Checks', 0, 'CheckID')).to eq('serfHealth')
+
+      instances = @client.health_list_service_instances(@service, filter: 'Checks.Status != "passing"')
+
+      expect(instances.length).to be 0
+    end
+  end
+end