chore: slightly improve error message on version mismatch (#211)

## Motivation

Users keep sending issues about unimplemented traits when they have
wrong versions, usually after `opentelemetry` is released and before
`tracing-opentelemetry` is.

## Solution

Reorder our bounds so that errors we control are shown first and add
`on_unimplemented` message and note.

The errors are still very long, but now the first one should be ours
(not just "type does not implement `opentelemetry::Tracer`") and we can
provide more information there. It's somewhat probable some people will
see the long errors and not read it at all, but this has the potential
to help at least some users.

Author: mladedav

Cargo.toml

@@ -30,6 +30,7 @@ tracing-core = "0.1.28"
 tracing-subscriber = { version = "0.3.0", default-features = false, features = ["registry", "std"] }
 tracing-log = { version = "0.2.0", default-features = false, optional = true }
 once_cell = "1.13.0"
+rustversion = "1.0.9"
 smallvec = { version = "1.0", optional = true }
 
 # Fix minimal-versions

src/tracer.rs

@@ -34,6 +34,14 @@ use opentelemetry_sdk::trace::{IdGenerator, Tracer as SdkTracer};
 /// [`OpenTelemetrySpanExt::set_parent`]: crate::OpenTelemetrySpanExt::set_parent
 /// [`OpenTelemetrySpanExt::context`]: crate::OpenTelemetrySpanExt::context
 /// [`Context`]: opentelemetry::Context
+#[rustversion::attr(
+    since(1.78),
+    diagnostic::on_unimplemented(
+        note = "Make sure you're using correct `opentelemetry` versions compatible with this \
+            `tracing-opentelemetry`. The `opentelemetry` versions are usually one version lower \
+            than `tracing-opentelemetry`."
+    )
+)]
 pub trait PreSampledTracer {
     /// Produce an otel context containing an active and pre-sampled span for
     /// the given span builder data.