Remove prometheus dependency and metrics (#769)

## Summary
- Remove the `prometheus` crate dependency from the workspace and
`xet_data`
- Delete `prometheus_metrics.rs` which defined 3 IntCounter metrics (CAS
bytes produced, bytes cleaned, bytes smudged)
- Remove metric increment calls from `file_upload_session.rs` and
`file_download_session.rs`
- Fix Windows CI flake: redb "Database already open" error in
`test_single_large`

These metrics were collected but never exposed via any HTTP endpoint or
text encoder, making them effectively dead code.

## Test plan
- [x] `cargo +nightly fmt` — clean
- [x] `cargo clippy --all-targets` — no new warnings
- [x] `cargo test -p xet-data` — 17/17 pass
- [x] `cargo test -p xet-data --features simulation --test
test_clean_smudge` — 14/14 pass (including `test_single_large`)
- [x] WASM builds (`hf_xet_wasm`, `hf_xet_thin_wasm`) — both succeed

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: this removes unused Prometheus metrics plumbing and related
dependencies without changing the core upload/download logic. Main risk
is loss of any downstream reliance on these counters at build time
(e.g., feature flags or imports).
> 
> **Overview**
> Removes the `prometheus` dependency from the workspace and `xet_data`,
and updates lockfiles accordingly (including WASM-related lockfiles).
> 
> Deletes `xet_data`’s `prometheus_metrics` module and strips the
associated counter increments from `FileUploadSession` and
`FileDownloadSession`, leaving the data processing behavior unchanged
aside from no longer recording these metrics.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c6c866b7ca. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

wasm/hf_xet_wasm/Cargo.lock

@@ -626,12 +626,6 @@ version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5baebc0774151f905a1a2cc41989300b1e6fbb29aff0ceffa1064fdd3088d582"
 
-[[package]]
-name = "fnv"
-version = "1.0.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
-
 [[package]]
 name = "foldhash"
 version = "0.1.5"
@@ -1594,41 +1588,6 @@ dependencies = [
  "unicode-ident",
 ]
 
-[[package]]
-name = "prometheus"
-version = "0.14.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3ca5326d8d0b950a9acd87e6a3f94745394f62e4dae1b1ee22b2bc0c394af43a"
-dependencies = [
- "cfg-if 1.0.4",
- "fnv",
- "lazy_static",
- "memchr",
- "parking_lot",
- "protobuf",
- "thiserror 2.0.18",
-]
-
-[[package]]
-name = "protobuf"
-version = "3.7.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d65a1d4ddae7d8b5de68153b48f6aa3bba8cb002b243dbdbc55a5afbc98f99f4"
-dependencies = [
- "once_cell",
- "protobuf-support",
- "thiserror 1.0.69",
-]
-
-[[package]]
-name = "protobuf-support"
-version = "3.7.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3e36c2f31e0a47f9280fb347ef5e461ffcd2c52dd520d8e216b52f93b0b0d7d6"
-dependencies = [
- "thiserror 1.0.69",
-]
-
 [[package]]
 name = "quinn"
 version = "0.11.9"
@@ -3593,7 +3552,6 @@ dependencies = [
  "itertools",
  "lazy_static",
  "more-asserts",
- "prometheus",
  "rand 0.9.2",
  "serde",
  "serde_json",