add an agent user-agent to APM server connections

[bryce-b]

In order to help debugging and gathering usage statistics, agents should use one of the following values for the User-Agent HTTP header:

• Header value should start with agent github repository as prefix and version apm-agent-${language}/${agent.version}.
• If both service.name and service.version are set, append (${service.name} ${service.version})
• If only service.name is set, append (${service.name})

An executable gherkin specification is also provided in user_agent.feature.

Examples:

• apm-agent-java/v1.25.0
• apm-agent-ruby/4.4.0 (myservice)
• apm-agent-python/6.4.0 (myservice v42.7)

reference spec : https://github.com/elastic/apm/blob/main/specs/agents/transport.md