Add guidelines on User-Agent header content

[MrAlias]

The New Relic SDK specification defines functionality to append field-values to the User-Agent header. Exporters need to use this functionality to include information about what systems they are instrumenting. This is critical to ensuring analytics about data sources are possible.

The specification should have a section providing guidelines about how this field is formed and what valid values include. At a minimum values need to conform to the product definitions defined in RFC 7231 Section 5.5.3. Additionally, values should likely be required to include information about the instrumented systems in some encouraged format.

[MrAlias]

Initial thoughts on what the guidelines should be changed to include:

• SHOULD append a product identifier for the instrumented system. (going with SHOULD here to allow for the end user to deliberately override the decision).
• MUST use the functionality offered by the SDK to append the User-Agent header. (don’t want the exporter overriding the SDK portion of the header).
• MUST append a valid value as defined by a product in RFC 7231 Section 5.5.5
• SHOULD include the name of the instrumented system.
• Then product name SHOULD remain as stable a value as possible.
• SHOULD include the version of the instrumented system if one is known.