Convert span attribute values to strings

[garthk]

As reported in opencensus-beam/opencensus_absinthe#10:

DD: Unable to send spans, DD reported an error: 400: 'json: cannot unmarshal number into Go struct field Span.meta of type string\n'

If DataDog can't cope with numbers, I think we'd better fix that in our DataDog adapters, because it doesn't make sense to change the rest of the OpenCensus ecosystem to cast attributes to strings just in case the spans might get sent to DataDog.

Nailing it down a little:

attribute_value() = any()
– opencensus Erlang typespec

AttributeValue … oneof … TruncatableString …int64 … bool_value … double_value
– OpenCensus protocol definition

A span contains a SpanContext and allows users to record tracing events based on the data model defined here
– OpenCensus specification

[garthk]

@sneako, it occurs to me another good point to get this fixed is at DataDog's end. Their API reference says only:

a dictionary of key-value metadata. e.g. tags

… and doesn't express much about what types the values might be. Perhaps they're open to casting all the values they get to strings at receipt time?

[hauleth]

I think it should be fixed in DD adapter.

[sneako]

@hauleth When you say 'DD adapter' are you referring to the Datadog agent, or this library?

[hauleth]

This library.

[hauleth]

@sneako DD probably should fix either their agent or documentation as well, but we do not have control over these. Fixing it there is better solution for now.

[sneako]

I have also opened a ticket with Datadog's support team to improve their documentation or the agent. Thanks everyone!

[sneako]

Datadog has updated their docs as well DataDog/documentation#4609