Update gateway to accept OTLP headers as string (spec v1.13.0)

[lpcox]

Context

The MCP Gateway Specification is being updated to v1.13.0 in github/gh-aw#25135. The headers field in the opentelemetry configuration is changing from object | string to string-only. The compiler will now emit "headers": "${OTEL_EXPORTER_OTLP_HEADERS}" as a raw string value instead of converting it to a JSON object.

What Needs to Change

1. Stdin config: accept headers as a string

StdinOpenTelemetryConfig.Headers in internal/config/config_stdin.go is currently map[string]string. It needs to become a string:

// Before
Headers map[string]string `json:"headers,omitempty"`

// After  
Headers string `json:"headers,omitempty"`

2. TOML config: align if needed

Check TracingConfig / OpenTelemetryConfig in internal/config/config_tracing.go — if it also has Headers map[string]string, update to string.

3. Tracing provider: parse the string at export time

internal/tracing/provider.go currently passes headers as map[string]string to otlptracehttp.WithHeaders(). It will need to parse the key=value,key=value string format into a map before passing to the OTLP exporter:

func parseOTLPHeaders(raw string) map[string]string {
    headers := make(map[string]string)
    for _, pair := range strings.Split(raw, ",") {
        k, v, ok := strings.Cut(strings.TrimSpace(pair), "=")
        if ok {
            headers[strings.TrimSpace(k)] = strings.TrimSpace(v)
        }
    }
    return headers
}

4. JSON schema: update to string type

Update internal/config/schema/mcp-gateway-config.schema.json — change opentelemetry.headers from object with additionalProperties: string to type: string.

5. Update tests

• Config parsing tests for string headers
• Tracing provider tests for header string parsing
• Validation tests if any check header types

References

• fix(spec+compiler): update MCP gateway OTLP headers to string-only (v1.13.0) gh-aw#25135 — spec + compiler PR (v1.13.0)
• Spec §4.1.3.6 — OpenTelemetry Configuration