Decode URL-encoded headers in environment vars

The [opentelemetry-specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specifying-headers-via-environment-variables) wants `OTEL_EXPORTER_OTLP_HEADERS` to conform to the [Baggage HTTP Header Format](https://github.com/w3c/baggage/blob/main/baggage/HTTP_HEADER_FORMAT.md).

To effectively use the headers from `OTEL_EXPORTER_OTLP_HEADERS` for
things like authentication (a common use case) the values need to be
decoded.

Given a header like this:

```
export OTEL_EXPORTER_OTLP_HEADERS="Authorization=Basic base64"
```

The code prior to this commit would reject the header for containing
whitespace mid-value.

If the header is modified to this:

```
export OTEL_EXPORTER_OTLP_HEADERS="Authorization=Basic%20base64"
```

The code prior to this commit would send the header as `Authorization:
Basic%20base64`, which will be rejected upstream.

This change applies `urllib.parse.unquote` to header **names** and
**values**, based on precedent set by the following libraries:

- [opentelemetry-go](https://github.com/open-telemetry/opentelemetry-go)
- [opentelemetry-js](https://github.com/open-telemetry/opentelemetry-js)
- [opentelemetry-ruby](https://github.com/open-telemetry/opentelemetry-ruby)

opentelemetry-api/src/opentelemetry/util/re.py

@@ -15,6 +15,7 @@
 import logging
 from re import compile, split
 from typing import Mapping
+from urllib.parse import unquote
 
 _logger = logging.getLogger(__name__)
 
@@ -51,8 +52,8 @@ def parse_headers(s: str) -> Mapping[str, str]:
             continue
         # value may contain any number of `=`
         name, value = match.string.split("=", 1)
-        name = name.strip().lower()
-        value = value.strip()
+        name = unquote(name.strip()).lower()
+        value = unquote(value.strip())
         headers[name] = value
 
     return headers

opentelemetry-api/tests/util/test_re.py

@@ -34,6 +34,11 @@ def test_parse_headers(self):
             # different header values
             ("name=", [("name", "")], False),
             ("name===value=", [("name", "==value=")], False),
+            # url-encoded headers
+            ("key=value%20with%20space", [("key", "value with space")], False),
+            ("key%21=value", [("key!", "value")], False),
+            # header name case normalization
+            ("Key=Value", [("key", "Value")], False),
             # mix of valid and invalid headers
             (
                 "name1=value1,invalidName, name2 =   value2   , name3=value3==",