fix: ensure correct run-context for 'memcached' instrumentation

[trentm]

This ensures that a created 'memcached' span is not the currentSpan
in user code after the call.

Refs: #2430

Checklist

• Implement code
• Add tests
• Add CHANGELOG.asciidoc entry
• TAV=memcached run

[apmmachine]

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• Start Time: 2021-12-01T21:37:35.119+0000

• Duration: 21 min 33 sec

• Commit: 2dcb7ea

Test stats 🧪

Test	Results
Failed	0
Passed	22
Skipped	0
Total	22

🤖 GitHub comments

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[trentm]

run module tests for memcached

[trentm]

run module tests for memcached

[trentm]

https://apm-ci.elastic.co/job/apm-agent-nodejs/job/apm-agent-nodejs-mbp/view/change-requests/job/PR-2475/

[trentm]

TAV=memcached test run passed: https://apm-ci.elastic.co/blue/organizations/jenkins/apm-agent-nodejs%2Fapm-agent-nodejs-mbp/detail/PR-2475/4/pipeline

[astorm]

👍 Approving. diff was a little gnarly, but it looks like it does the thing.

[astorm]

nit: it doesn't look like the function ever uses _server

[trentm]

Yah, a _ prefix on args is my sometimes way of saying "this is the expected position of this argument in the call signature, but I'm not using it". This _ prefix pattern is this eslint rule (typescript-eslint/typescript-eslint#1510) which I believe is a default in standards set of eslint style rules.

Happy to drop it if you prefer. For trailing args it is purely for self-documenting. For non-trailing args (e.g.

apm-agent-nodejs/lib/noop-transport.js

Line 28 in 3928f8d

sendError (_error, cb) {

) it is more helpful/necessary.