feat: generate User-Agent for transport per APM spec

[trentm]

https://github.com/elastic/apm/blob/master/specs/agents/transport.md#user-agent
This adds serviceName and serviceVersion to the User-Agent.

Closes: #2364

Checklist

• Implement code
• Add tests
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

[trentm]

elastic/apm-nodejs-http-client#169 was the related PR for the http-client (for v10.2.0).

[trentm]

BTW, if serviceVersion was not sanitized and non-ISO-8859-1 chars (or really, non-8-bit chars, or really chars that don't match node's internal const headerCharRegex = /[^\t\x20-\x7e\x80-\xff]/; used for header value validation, then requests to APM server would error out with:

TypeError [ERR_INVALID_CHAR]: Invalid character in header content ["User-Agent"]

[apmmachine]

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• Start Time: 2021-10-28T21:16:55.064+0000

• Duration: 20 min 15 sec

• Commit: f4d93a2

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Test	Results
Failed	0
Passed	22
Skipped	0
Total	22

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