Update spec-compliance-matrix.md with Rust env var sampler support (#4795)

As discussed in this issue, the Rust SDK does support the
`OTEL_TRACES_SAMPLER` and `OTEL_TRACES_SAMPLER_ARG` environment
variables:
open-telemetry/opentelemetry-rust#3283

## Changes

Updates the compliance table. I have not verified any of the unchanged
cells, so it's possible there are other env vars marked "unsupported"
that Rust supports.

For non-trivial changes, follow the [change proposal
process](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CONTRIBUTING.md#proposing-a-change).

* [ ] Related issues #
* [ ] Related [OTEP(s)](https://github.com/open-telemetry/oteps) #
* [ ] Links to the prototypes (when adding or changing features)
* [ ]
[`CHANGELOG.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CHANGELOG.md)
file updated for non-trivial changes
* [x] [Spec compliance
matrix](https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix/template.yaml)
updated if necessary

Author: BatmanAoD

spec-compliance-matrix.md

@@ -273,8 +273,8 @@ Note: Support for environment variables is optional.
 | OTEL_LINK_ATTRIBUTE_COUNT_LIMIT | + | - |  | + | + | + | + |  | - | + |  |
 | OTEL_LOGRECORD_ATTRIBUTE_COUNT_LIMIT | + |  |  |  | + |  | + |  | - |  |  |
 | OTEL_LOGRECORD_ATTRIBUTE_VALUE_LENGTH_LIMIT | + |  |  |  | + |  | + |  | - |  |  |
-| OTEL_TRACES_SAMPLER | + | + | + | + | + | + | + | - | - | - |  |
-| OTEL_TRACES_SAMPLER_ARG | + | + | + | + | + | + | + | - | - | - |  |
+| OTEL_TRACES_SAMPLER | + | + | + | + | + | + | + | + | - | - |  |
+| OTEL_TRACES_SAMPLER_ARG | + | + | + | + | + | + | + | + | - | - |  |
 | OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT | + | + | + | + | + | - | + |  | - | + |  |
 | OTEL_ATTRIBUTE_COUNT_LIMIT | + | + | + | + | + | - | + |  | - | + |  |
 | OTEL_METRIC_EXPORT_INTERVAL | + | + |  | + | + |  | + |  | - | + |  |

spec-compliance-matrix/rust.yaml

@@ -480,9 +480,9 @@ sections:
       - name: OTEL_LOGRECORD_ATTRIBUTE_VALUE_LENGTH_LIMIT
         status: '?'
       - name: OTEL_TRACES_SAMPLER
-        status: '-'
+        status: '+'
       - name: OTEL_TRACES_SAMPLER_ARG
-        status: '-'
+        status: '+'
       - name: OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT
         status: '?'
       - name: OTEL_ATTRIBUTE_COUNT_LIMIT