[studio][Bug] Proxy OTLP header parsing can reject valid values and expose secrets

### Before Creating the Bug Report

- [x] I found a bug, not just a question.
- [x] I searched existing issues and pull requests for metricsGrpcExporterHeader, OTLP header parsing, and exporter-header secret leakage.
- [x] I confirmed the affected code belongs to this repository.

### Describe the Bug

ProxyMetricsManager#start parses metricsGrpcExporterHeader entries with item.split(:).

This has two related problems:

1. A valid header value containing a colon (for example a URI or a structured authorization value) produces more than two segments and is discarded.
2. When parsing fails, the warning logs the complete header configuration. That string may contain authorization tokens, API keys, or other credentials.

The same broad split behavior is also used for metricsLabel, where a label value containing a colon is unnecessarily rejected.

### Expected Behavior

- Split each entry at the first colon only.
- Trim and require a non-empty key.
- Preserve the remainder of the value, including additional colons.
- Warn about an invalid entry without logging the complete configured labels or headers.
- Add focused unit tests for colon-containing values, empty keys, malformed entries, and secret-safe diagnostics.

### Affected Area

RocketMQ Proxy metrics initialization / OTLP exporter configuration. This is relevant to RocketMQ Studio deployments that use Proxy metrics for observability.

### Proposed Scope

Keep the change local to metrics key/value parsing and its tests. Do not change exporter defaults, protocols, or public APIs.

Start at ProxyMetricsManager#start and trace parsing for metricsGrpcExporterHeader and metricsLabel. Add focused unit coverage for colon-containing values, empty keys, malformed entries, and diagnostics that do not expose configured secrets; done means valid remainder values are preserved and invalid entries are warned about safely.

Written by the indexing model from the issue text.