perf(fetch): avoid allocations in client_scope for borrowed-'static names (#514)

`client_scope` previously called `to_string()` on the runtime and
transport names, forcing a heap allocation on every client build even
though the transport already stores these names as `Cow<'static, str>`.

`client_scope` now takes `impl Into<Value>` for the runtime and
transport names — the exact bound `KeyValue::new` requires — and hands
each value straight into the instrumentation scope attributes with no
extra clone inside the function. The production call sites in
`client_builder.rs` pass
`transport.runtime().clone()`/`transport.name().clone()`; cloning the
transport's `Cow<'static, str>` is allocation-free for the common
borrowed-`'static` case and simply hands over ownership. The in-file
unit tests pass plain `&'static str` literals, which likewise convert
into `Value` without allocating.

Net effect: borrowed-`'static` runtime/transport names no longer
allocate when building the meter scope, and the signature stays
idiomatic (no `clippy::ptr_arg` `#[expect]` or `&Cow` borrow needed).

Verified with `just package=fetch clippy` (clean) and `just
package=fetch test` (153 passed).

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sandersaares

crates/fetch/src/client_builder.rs

@@ -56,7 +56,7 @@ impl HttpClientBuilder {
         Self {
             options: ClientOptions::default(),
             pipeline_builder: PipelineBuilder::default(),
-            metering: Metering::global(client_scope(transport.runtime(), transport.name())),
+            metering: Metering::global(client_scope(transport.runtime().clone(), transport.name().clone())),
             transport,
             resilience_context: HttpResilienceContext::new(&clock).name(DEFAULT_HTTP_CLIENT_NAME).use_logs(),
         }
@@ -241,7 +241,10 @@ impl HttpClientBuilder {
     pub fn meter_provider(self, meter_provider: &dyn MeterProvider) -> Self {
         // Update the metering at all relevant places
         Self {
-            metering: Metering::custom(meter_provider, client_scope(self.transport.runtime(), self.transport.name())),
+            metering: Metering::custom(
+                meter_provider,
+                client_scope(self.transport.runtime().clone(), self.transport.name().clone()),
+            ),
             resilience_context: self.resilience_context.use_metrics(meter_provider),
             ..self
         }

crates/fetch/src/telemetry.rs

@@ -98,11 +98,11 @@ impl From<Metering> for Meter {
 
 /// Builds the `fetch` instrumentation scope carrying the `fetch.runtime` and
 /// `fetch.transport` attributes, attached to every metric a client records.
-pub(crate) fn client_scope(runtime: &str, transport: &str) -> InstrumentationScope {
+pub(crate) fn client_scope(runtime: impl Into<Value>, transport: impl Into<Value>) -> InstrumentationScope {
     InstrumentationScope::builder(METER_NAME)
         .with_attributes([
-            KeyValue::new(FETCH_RUNTIME_ATTRIBUTE, runtime.to_string()),
-            KeyValue::new(FETCH_TRANSPORT_ATTRIBUTE, transport.to_string()),
+            KeyValue::new(FETCH_RUNTIME_ATTRIBUTE, runtime),
+            KeyValue::new(FETCH_TRANSPORT_ATTRIBUTE, transport),
         ])
         .build()
 }