Explain why VirtualFile stores tenant_id and timeline_id as strings (#4930)

## Problem

One might wonder why the code here doesn't use `TimelineId` or
`TenantId`. I originally had a refactor to use them, but then discarded
it, because converting to strings on each time there is a read or write
is wasteful.

## Summary of changes

We add some docs explaining why here no `TimelineId` or `TenantId` is
being used.

pageserver/src/virtual_file.rs

@@ -53,6 +53,9 @@ pub struct VirtualFile {
     pub path: PathBuf,
     open_options: OpenOptions,
 
+    // These are strings becase we only use them for metrics, and those expect strings.
+    // It makes no sense for us to constantly turn the `TimelineId` and `TenantId` into
+    // strings.
     tenant_id: String,
     timeline_id: String,
 }