feat: add provider_init_rate to telemetry

[nicklasl]

Summary

• Adds ProviderInitRate message to TelemetryData proto (field 9) with count and map<string, string> labels
• On the first telemetry flush after provider initialization, each provider includes a ProviderInitRate{count: 1, labels: {encryption: "true"/"false"}} entry
• Subsequent flushes carry no init data
• WASM-based providers (JS, Java, Go, Python) deserialize the flush output, add the entry, and re-serialize — only on the first flush
• Rust native provider sets it directly on TelemetryData

Companion backend PR: https://ghe.spotify.net/konfidens/epx-flags-resolver/pull/1699

Test plan

• JS demo app verified: first flush contains provider_init_rate with correct labels, subsequent flushes do not
• Run provider test suites (make test per provider)
• Docker build (docker build .)
• Verify confidence_resolver_sdk_provider_init_rate_total metric appears in VictoriaMetrics after backend PR lands

🤖 Generated with Claude Code

[vahidlazio]

other providers push but here we replace, can we use decoded.telemetryData!.providerInitRate.push({ count: 1, labels: this.initLabels }); to be consistent?

[vahidlazio]

we didn't throw before, means failures were silently dropped, did we check that all callers handle this?

[vahidlazio]

also, like other providers, js must also have some emit if not send in onClose i think like:

async onClose(): Promise<void> {
  await this.flush(timeoutSignal(3000));
  // Guarantee init telemetry even if flush() failed or returned empty
  if (this.initTelemetryState !== 'sent') {
    try {
      const request = WriteFlagLogsRequest.encode(
        WriteFlagLogsRequest.create({
          telemetryData: {
            sdk: { id: SdkId.SDK_ID_JS_LOCAL_SERVER_PROVIDER, version: VERSION },
            providerInitRate: [{ count: 1, labels: this.initLabels }],
          },
        }),
      ).finish();
      await this.sendFlagLogs(request, timeoutSignal(3000));
      this.initTelemetryState = 'sent';
    } catch {
      // best-effort — provider is shutting down
    }
  }
  this.main.abort();
}

[vahidlazio]

nit: go and java always replace