OpenCensus is a vendor-agnostic single distribution of libraries to provide metrics collection and tracing for your services.
Metrics are any quantifiable piece of data that you would like to track, such as latency in a service or database, request content length, or number of open file descriptors. Viewing graphs of your metrics can help you understand and gauge the performance and overall quality of your application and set of services.
Traces show you how a request propagates throughout your application or set of services. Viewing graphs of your traces can help you understand the bottlenecks in your architecture by visualizing how data flows between all of your services.