Use specific JSON::ParserError rescue instead of catch-all

Author: yuriadam-sap

src/bosh-director/lib/bosh/director/metrics_collector.rb

@@ -154,7 +154,12 @@ def fetch_and_update_gauge(endpoint, gauge)
         response = Net::HTTP.get_response('127.0.0.1', endpoint, @config.health_monitor_port)
         return unless response.is_a?(Net::HTTPSuccess)
 
-        deployment_counts = JSON.parse(response.body) rescue nil
+        begin
+          deployment_counts = JSON.parse(response.body)
+        rescue JSON::ParserError => e
+          @logger.warn("Failed to parse JSON response from #{endpoint}: #{e.message}")
+          return
+        end
         return unless deployment_counts.is_a?(Hash)
 
         existing_deployment_names = gauge.values.map do |key, _|

src/bosh-director/spec/unit/bosh/director/metrics_collector_spec.rb

@@ -355,14 +355,21 @@ def tick
         end
 
         context 'when the health monitor returns a non-json response' do
+          let(:logger) { double(Logging::Logger) }
+          
           before do
+            allow(config).to receive(:metrics_server_logger).and_return(logger)
+            allow(logger).to receive(:info)
             stub_request(:get, "127.0.0.1:12345/unresponsive_agents")
-              .to_return(status: 200, body: JSON.dump("bad response"))
+              .to_return(status: 200, body: "not valid json {")
             stub_request(:get, "127.0.0.1:12345/unhealthy_agents")
-              .to_return(status: 200, body: JSON.dump("bad response"))
+              .to_return(status: 200, body: "not valid json {")
           end
 
-          it 'does not emit the vm metrics' do
+          it 'does not emit the vm metrics and logs a warning' do
+            expect(logger).to receive(:warn).with(/Failed to parse JSON response from \/unresponsive_agents/)
+            expect(logger).to receive(:warn).with(/Failed to parse JSON response from \/unhealthy_agents/)
+            
             metrics_collector.start
             metric = Prometheus::Client.registry.get(:bosh_unresponsive_agents)
             expect(metric.values).to be_empty