Add fs module support

[luismiramirez]

All fs module usage is now automatically instrumented.

Tests are in the express-redis sample application.

Automatic instrumentation is manually suppresed in the test span processor because it uses the fs module to handle the spans.json file. It is done using the supressTracing helper provided by OpenTelemetry core library.

Part of #763.

[unflxw]

I have looked into the issue where the Prisma test applications fail to boot when the FS instrumentation is added.

TL;DR: We gotta wait for a release containing the changes in this PR to be published (should be included in the next release) and update the @opentelemetry/instrumentation-fs package to that version.

The core of the issue is that the Prisma client calls util.promisify(fs.exists). It turns out that fs.exists is a bit unusual. It's deprecated by the Node team, because its callback signature does not follow the standard (err, result) => { ... } signature of Node callbacks. Instead, since fs.exists can't fail, its signature is just (result) => { ... }.

Since util.promisify expects the first argument of the callback to be an error, this means that promisifying fs.exists would result on the promise always being rejected, with the actual result being the rejection error.

This is consistent with the error message we see:

error2.clientVersion = this.client._clientVersion;

TypeError: Cannot create property 'clientVersion' on boolean 'true'

Prisma is trying to set the clientVersion property on the error object it received, but the error object isn't an object, but a boolean true, the return value of fs.exists, turned into the rejection value of a promise by util.promisify.

Now, the reason why this bug only happens for the instrumented version of the function is that the original fs.exists provides a custom promisified implementation of itself. If there is an util.promisify.custom symbol as a property of the function, util.promisify will use its value as the promisified implementation. The original fs.exists provides this, but the OpenTelemetry-instrumented version of the function doesn't.

Or rather, it doesn't yet, as there's a merged PR implementing this custom behaviour for the instrumented version of the function. This should fix our problems; we just need to wait for a release including the change.

I have not been able to verify the actual fix directly (npm's "install from GitHub" does not play well with monorepos) but I have confirmed that manually excluding fs.exists from being instrumented by commenting the relevant line of code fixes the issue, so I expect the fix above to also fix the issue.

[unflxw]

Changes look good, but let's not merge this until there's an updated version of the @opentelemetry/instrumentation-fs library, or else it will break for Prisma users.