content/faq: add trouble-shooting for none visible traces and none available metrics

[odeke-em]

It occured to me some months ago that there is a bit of a cognitive load when getting started with OpenCensus. For example, users would go through the labour of instrumenting their code and at the end wouldn't see traces nor metrics.

My response rubric was:

• So you started and stopped spans? e.g. ensure that span.end or span.close was successfully invoked?
• Did you create and configure the necessary exporters?
• With your exporters well created and configured, did you invoke trace.RegisterExporter or stats.RegisterExporter?
• With the exporters registered, perhaps did you change the sampling rate? By default it is 1/1000 or so, so for immediate examination of output, perhaps use the AlwaysSample sampler. For metrics, please change the report period

I think providing such a rubric translated to the various languages would massively help folks get their questions self answered and enable other developers.