chore(gatsby): add environment variable for setting tracing config file

[rudouglas]

Description

Currently, the only way to enable tracing is to pass the --open-tracing-config-file flag as part of the gatsby build/develop commands.

Because Gatsby Cloud just seems to run gatsby build and ignores these flags in the package.json, it's not possible to enable tracing on Gatsby Cloud

This enables tracing by allowing a user to set GATSBY_OPEN_TRACING_CONFIG_FILE instead

[jpvajda]

I was just following up on his PR review. if someone could let us know when it may be actioned that be cool! - Than you

[jpvajda]

Gatsby confirmed today @rudouglas

The tracing PR that you made is being addressed in the current sprint this week!

[jpvajda]

Is this PR still on track to be merged in by Aug 31st?

[KyleAMathews]

This is great!

Pushed a couple of updates to standardize the env var name & document it.

I've been thinking/working on tracing support a fair bit this week. Just posted a discussion topic @ #33145 & have put up some PRs:

• feat(gatsby-source-drupal): Add tracing for full/delta fetches and http requests #33142
• fix(gatsby): Assign parentSpan to activities that were missing them #33122
• fix(gatsby): fix nesting of tracing spans + add docs for OpenTelemetry #33098

Would love y'all's thoughts on improving Gatsby's observability support.

[rudouglas]

@KyleAMathews amazing thanks for looking at this, and thanks for the fresh PR's they look awesome and will really help get more valuable data from tracing. We would love to collaborate more and making this even more valuable 🥇

[jpvajda]

@KyleAMathews Thank you!