Review sampling relevant flag for attributes that are part of upcoming HTTP semconv stability

[trask]

(related Azure/azure-sdk-for-java#26988 (comment))

[trask]

Existing sampling_relevant attributes:

HTTP client spans

• http.request.method
• server.address
• server.port
• url.full

HTTP server spans

• http.request.method
• server.address
• server.port
• url.path
• url.query
• url.scheme

Potential additional sampling_relevant attributes:

HTTP server spans

• user_agent.original - I think ok to assume headers are available on span start (we already do this for server.address which is based on host header)
• http.request.header.<key> - same
• client.address - seems useful for sampling from a specific client or address block