Stabilize span attributes customization: builder API shape

[cijothomas]

Context

The experimental_span_attributes feature in opentelemetry-appender-tracing
is ready to be stabilized. Today its only API is:

.with_span_attribute_allowlist(["session.id"])  // requires cargo feature

When we drop the cargo feature, we also need a way to enable the
behavior at runtime — not every user wants to pay per-span overhead.

Two builder shapes under consideration. Pick one.

---

Option A — two methods, allowlist implies enable

.with_span_attributes(true)                       // enable, copy all
.with_span_attribute_allowlist(["session.id"])    // implies enable + filter

• Two methods; subtle interaction if both called.
• bool argument enables conditional config: b.with_span_attributes(cfg.enable).

Option B — one method, allowlist semantics

.with_span_attributes([])                         // enable, copy all
.with_span_attributes(["session.id"])             // enable, allowlist
// (not called)                                   // disabled

• Single method, no boolean trap.
• Empty iterator = "enable all" sentinel — slight cognitive load.
• Conditional enable requires if cfg.enable { b = b.with_span_attributes(...) }.

---

In both options:

• Default is disabled (no per-span work).
• Last call wins if invoked twice.

[cijothomas]

@codefromthecrypt Can you share your thoughts.

[cijothomas]

@leghadjeu-christian Can you share you thoughts?

[codefromthecrypt]

I like option b! I also do not think this should be enabled by default.

[cijothomas]

Going with a variation of Option B (suggested by @utpilla ) — single with_span_attributes method that accepts a SpanAttributes value type with named constructors:

use opentelemetry_appender_tracing::layer::SpanAttributes;

// Copy all tracing-span attributes:
let layer = OpenTelemetryTracingBridge::builder(&provider)
    .with_span_attributes(SpanAttributes::all())
    .build();

// Copy only an allowlist:
let layer = OpenTelemetryTracingBridge::builder(&provider)
    .with_span_attributes(SpanAttributes::allowlist(["session.id"]))
    .build();

This avoids the "empty iterator = copy all" sentinel from Option B and eliminates the two-knob misuse surface from Option A (no invalid states are expressible). One setter, so "what if both are called?" simply cannot happen.

PR #3482 has been updated with this design.

[cijothomas]

#3482 closest