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

docs/docs/performance-tracing.md

@@ -33,7 +33,7 @@ The configuration file is a JavaScript file that exports two functions: `create`
 
 ### 3. Start Gatsby with tracing turned on
 
-The above configuration file can be passed to Gatsby with the `--open-tracing-config-file` command-line option. When Gatsby is started with this option, it will load the supplied tracing configuration file, and call its `create` function. The returned Tracer will be used for tracing the build. Once the build has stopped, the configuration file's `stop` method will be called, allowing the tracing implementation to perform any cleanup.
+The above configuration file can be passed to Gatsby with the `--open-tracing-config-file` command-line option or an environment variable named `GATSBY_OPEN_TRACING_CONFIG_FILE`. When Gatsby is started with this option, it will load the supplied tracing configuration file, and call its `create` function. The returned Tracer will be used for tracing the build. Once the build has stopped, the configuration file's `stop` method will be called, allowing the tracing implementation to perform any cleanup.
 
 ## Tracing backend examples
 
@@ -108,7 +108,7 @@ exports.stop = async () => {
    we run Gatsby in a special way telling Node to require our tracing file immediately.
 
 ```shell
-node -r ./tracing.js node_modules/gatsby/cli.js build --open-tracing-config-file tracing.js
+GATSBY_OPEN_TRACING_CONFIG_FILE=tracing.js node -r ./tracing.js node_modules/gatsby/cli.js build
 ```
 
 ### Local Jaeger with Docker

packages/gatsby/src/commands/build.ts

@@ -75,7 +75,9 @@ module.exports = async function build(program: IBuildArgs): Promise<void> {
   markWebpackStatusAsPending()
 
   const publicDir = path.join(program.directory, `public`)
-  initTracer(program.openTracingConfigFile)
+  initTracer(
+    process.env.GATSBY_OPEN_TRACING_CONFIG_FILE || program.openTracingConfigFile
+  )
   const buildActivity = report.phantomActivity(`build`)
   buildActivity.start()
 

packages/gatsby/src/commands/develop-process.ts

@@ -101,7 +101,9 @@ module.exports = async (program: IDevelopArgs): Promise<void> => {
     process.exit(0)
   })
 
-  initTracer(program.openTracingConfigFile)
+  initTracer(
+    process.env.GATSBY_OPEN_TRACING_CONFIG_FILE || program.openTracingConfigFile
+  )
   markWebpackStatusAsPending()
   reporter.pendingActivity({ id: `webpack-develop` })
   telemetry.trackCli(`DEVELOP_START`)