Default newrelic-tracing instance doesn't work with Istio 1.4.3

[cmanzi]

Describe the bug
Using the default install of the istio adapter with Istio 1.4.3 produces the following error in Mixer:

rule='newrelic-tracing.rule.istio-system'.Match: OR(INDEX($request.headers, "x-b3-sampled"), "0") arg 2 ("0") typeError got DURATION, expected STRING

Thus preventing the adapter from receiving trace spans from Mixer.

Changing the "0" in the 'newrelic-tracing' rule to "" in the following location resolves this issue:

...((request.headers["x-b3-sampled"] | "") == "1")

With this change to the rules I can see trace spans populating in New Relic One now.

This seems to be an issue with the second argument to the OR function - "0" - being interpreted as a 'Duration' rather than a 'String' (which is the type of the first argument in this case) in Mixer's rule parsing code: https://github.com/istio/istio/blob/master/mixer/pkg/lang/ast/expr.go#L232-L251

To Reproduce
Steps to reproduce the behavior:

1. Install Istio 1.4.3
2. Install newrelic-istio-adapter with default rules and tracing enabled
3. generate trace spans, look in error logs for Mixer (telemetry)

Expected behavior
Istio can send trace spans to the adapter with the default rules

Version information (please complete the following information):

• newrelic-istio-adapter: 2.0.1
• Kubernetes: 1.15.7
• Istio: 1.4.3

Additional context
Add any other context about the problem here.

[MrAlias]

Thanks for the heads up, and fix suggestion. It looks like the way to go.

I should be able to have a PR ready in a bit to address this.