docs: explain quoted Unicode metric names (#2571)

## Summary

Clarify how metric names outside the legacy character set are
represented when `escaping=allow-utf-8` is used with the Prometheus and
OpenMetrics 1.0 text formats.

The documentation now:

- Explains that non-legacy metric names must use quoted syntax.
- Clarifies why quoted metric names appear inside braces.
- Includes an example using a Unicode counter name.
- Links to the complete Prometheus exposition format specification.

The reported output is valid text-format syntax rather than an encoding
error.

Closes #1576.

## Testing

NA

Signed-off-by: Arnab Nandy <arnab_nandy7@yahoo.com>

Author: arnabnandy7

docs/content/exporters/unicode.md

@@ -26,6 +26,19 @@ When `escaping=allow-utf-8` is passed, add valid UTF-8 characters to the metric
 without replacing them. This allows you to use dots in metric and label names, as well as
 other UTF-8 characters, without any replacements.
 
+Names outside the legacy character set must use the quoted syntax in the Prometheus and
+OpenMetrics 1.0 text formats. A quoted metric name is written inside braces, followed by its
+labels. For example, a counter named `test_我喜欢茶` with a `test` label is exposed as:
+
+```text
+# TYPE "test_我喜欢茶" counter
+{"test_我喜欢茶_total",test="test_我喜欢茶"} 1.0
+```
+
+The braces do not turn the metric name into a label. They are the required text-format notation
+for a quoted metric name. See the [Prometheus exposition
+format](https://prometheus.io/docs/instrumenting/exposition_formats/) for the complete syntax.
+
 OpenMetrics 2.0 supports UTF-8 names natively, so when the OpenMetrics 2.0 writer serves a request
 that explicitly asks for `version=2.0.0`, names are not escaped unless the request also contains an
 `escaping` parameter.