Allow --duration=0 or infinite duration

[howardjohn]

It can be useful to continually apply load. For these cases, an infinite duration would be nice. Currently setting duration=0 provides an error:

$ docker run envoyproxy/nighthawk-dev:59683b759eb8f8bd8cce282795c08f9e2b3313d4 nighthawk_client google.com --duration 0
[2020-05-01 17:01:34.204][1][warning][misc] [external/envoy/source/common/protobuf/utility.cc:198] Using deprecated option 'nighthawk.client.CommandLineOptions.tls_context' from file options.proto. This configuration will be removed from Envoy soon. Please see https://www.envoyproxy.io/docs/envoy/latest/intro/deprecated for details.
[2020-05-01 17:01:34.204][1][debug][misc] [external/envoy/source/common/protobuf/utility.cc:255] Proto validation error; throwing Proto constraint validation failed (CommandLineOptionsValidationError.Duration: ["value must be greater than or equal to " "1ns"]): requests_per_second {

[oschaaf]

+1, by chance this came up in an unrelated meeting yesterday as well. It’s will be a fairly easy change to get this to work

[oschaaf]

I think doing this would make most sense when Nighthawk has obtained at least one of these capabilities:

• Handle graceful termination upon signal reception (First step is in review)
• Add functionality to flush metrics periodically

Arguably this could also have some use when one totally isn't interested in results, but I think more use cases would be added as the above functionalities become available.

[howardjohn]

The use case I was thinking was not caring about the results. For example measuring CPU usage of the server or relying on server-side telemetry. But that makes sense as well