chore: exclude otlp trace provider from slim builds (#16140)

This adds a build flag to `exporter.go` in `coderd/tracing` that skips
compiling the file in slim builds. This file brings in some relatively
hefty dependencies that were growing the size of the slim binary a few
megabytes. All files that import `exporter.go` also aren't included in
slim builds, so this is safe.

We end up saving 5.4MB on the Linux slim binary.

coderd/tracing/exporter.go

@@ -1,3 +1,5 @@
+//go:build !slim
+
 package tracing
 
 import (