Build Span Attributes which may be missing from OTEL-provided Span

[bradAtTraceable]

The Span that is create by the OTEL java agent may not have some of the attributes that are needed for properly evaluating the execution of a request. Specifically, the "http.url" attribute may be missing. This PR produces a Map of those Span attributes that are required, but may not exist, in the OTEL-provided Span. The contents of this possibly-missing attribute Map may then be used during the evaluation of the request execution.

Note that the possibly-missing attributes are not merged into the Span with this PR. Instead, they are provided as a separate (optional) argument to the Filter.evaluateRequestHeaders() and .evaluateRequestBody() methods.