internal/cmd/pdatagen: add support for specifying a work dir (#13890)

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#### Description

This PR adds a new flag (`-C path`) to the `pdatagen` utility, which
allows specifying the workdir for the tool.

Currently `pdatagen` expects to find the respective files in`pdata`
directory of the `$cwd`. When `pdatagen` is called outside of the
`$repo_root` the tool fails, because it cannot find `pdata` and panics.

This PR adds an optional `-C path` flag, which would allow `pdatagen` to
use an alternative workdir, and also be able to be called outside of the
`$repo_root` directory.

See #13844 for more details about this PR and the proposed changes. The
commit from this PR have been extracted from #13844 .

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#### Testing

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#### Documentation

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Author: dnaeon

internal/cmd/pdatagen/main.go

@@ -4,20 +4,31 @@
 package main
 
 import (
+	"flag"
+	"fmt"
+	"os"
+
 	"go.opentelemetry.io/collector/internal/cmd/pdatagen/internal/pdata"
 )
 
-func check(e error) {
+// checkErr prints the given error and exits when e is non-nil.
+func checkErr(e error) {
 	if e != nil {
-		panic(e)
+		fmt.Println(e)
+		os.Exit(1)
 	}
 }
 
 func main() {
+	var workdir string
+	flag.StringVar(&workdir, "C", ".", "set work directory")
+	flag.Parse()
+
+	checkErr(os.Chdir(workdir))
 	for _, fp := range pdata.AllPackages {
-		check(fp.GenerateFiles())
-		check(fp.GenerateTestFiles())
-		check(fp.GenerateInternalFiles())
-		check(fp.GenerateInternalTestsFiles())
+		checkErr(fp.GenerateFiles())
+		checkErr(fp.GenerateTestFiles())
+		checkErr(fp.GenerateInternalFiles())
+		checkErr(fp.GenerateInternalTestsFiles())
 	}
 }