Allow O(1) get operations for SpanBuilder::attributes [breaking] (#799)

* Use index_map::IndexMap instead of Vec<KeyValue> to store SpanBuilder's attributes.

* Test suite compiles.

* Rustfmt

* Wrap IndexMap to expose only the methods that are insertion-order preserving.

* Raise MSRV to 1.51 to get support for const generics.

* Fix doctest.

* Fix lint.

* Add specialised implementations to make it easier to work with KeyValue iterators/collections.

* Bump MSRV to get access to array::map.

* Minimise breakages for existing users.

* Fix invocation.

* Rustfmt

.github/workflows/ci.yml

@@ -68,7 +68,7 @@ jobs:
     - uses: actions-rs/toolchain@v1
       with:
         profile: minimal
-        toolchain: 1.49.0
+        toolchain: 1.55.0
         override: true
     - name: Run tests
       run: cargo --version &&

opentelemetry-api/Cargo.toml

@@ -11,6 +11,7 @@ lazy_static = "1.4"
 pin-project = { version = "1.0.2", optional = true }
 thiserror = "1"
 tokio-stream = { version = "0.1", optional = true }
+indexmap = "1"
 
 [package.metadata.docs.rs]
 all-features = true

opentelemetry-api/src/trace/mod.rs

@@ -168,13 +168,15 @@ use thiserror::Error;
 
 mod context;
 pub mod noop;
+mod order_map;
 mod span;
 mod span_context;
 mod tracer;
 mod tracer_provider;
 
 pub use self::{
     context::{get_active_span, mark_span_as_active, FutureExt, SpanRef, TraceContextExt},
+    order_map::OrderMap,
     span::{Span, SpanKind, Status},
     span_context::{SpanContext, SpanId, TraceFlags, TraceId, TraceState},
     tracer::{SamplingDecision, SamplingResult, SpanBuilder, Tracer},