Improve user-agent header

[watson]

The user agent was updated recently (#1238) from:

{package-name}/{package-version}

to:

{agent-name}/{package-version}

Following discussion, we're updating it once more to get the package-name back into the header:

{agent-name}/{package-version} {package-name}/{package-version}

I propose also that we call the agent-name elasticapm-nodejs instead of elasticapm-node while we're at it. The duplicate package-version is intentional (see discussion).

[trentm]

The user-agent was spec'd to be $repoName/$version ($serviceName $serviceVersion) at https://github.com/elastic/apm/blob/main/specs/agents/transport.md#user-agent

And that was implemented in #2402 a while back.