Support for "map" conventions based on prefixes

[Oberon00]

See this suggested semantic convention for HTTP headers: open-telemetry/opentelemetry-specification#1061.

Right now, I think it is not possible to specify this in the semantic convention generator. It would be useful however to generate a constant for the http.request.header prefix (or http.request.header. with a trailing dot, or a function string httpRequestHeaderName(string key) { return 'http.request.header' + key; }.

In the markdown, this should probably be designated by adding .* to the name of the attribute. E.g. like this:

Attribute	Type	Description	Examples	Required
http.request.header.<key>	string[]	HTTP request headers, <key> being the HTTP Header name (case preserving), the value being the header values.	http.request.header.Content-Type=["application/json"]; http.request.X-Forwarded-for=["1.2.3.4", "1.2.3.5"]	No

In the YAML source:

• The type could be specified like type: prefix_map: string[].
• Examples need to be handled specially here, as they need to include both the key (suffix) and a value.

[Oberon00]

Caveat: We should not implement this before discussing over in the spec repo if that prefix-based scheme is something we may want to use more often.

[lmolkova]

I believe it's fixed in the #186