add otel exporter option to override metric_reporting_period

[dhontecillas]

In case we have multiple OTEL metric exporters, we might want to tweak the reporting period for each one of them. (For example, if one of those is reporting to our on premises instance with a lower retention period, and another is sent to an external provider with longer retention period, and might be some rate limit).

With this PR, we add a custom_metric_reporting_period to the OTEL exporters config (docs about it here: https://www.krakend.io/docs/telemetry/opentelemetry/#exporters-otlp )

custom_metric_reporting_period : unsigned integer specifying the seconds to wait between sending report, 0 will not override the metric_reporting_period set for all metric exporters (https://www.krakend.io/docs/telemetry/opentelemetry/#metric_reporting_period)

In order to easily check the reporting periods, I've created two otel exporters with addresses pointing to non-existing services, so we can see an error in the log, each time it tries to report the metrics:

• one at localhost:5005 that should be reporting each 5 seconds (global configuration)
• one at localhost:5011 that should be reporting each 11 seconds (overriden configuration)

{
  "$schema": "https://www.krakend.io/schema/v3.json",
  "version": 3,
  "name": "My first API Gateway - KrakenD",
  "port": 8081,
  "host": ["http://my_service:8080"],
  "debug_endpoint": true,
  "extra_config": { 
    "telemetry/opentelemetry": {
      "service_name": "krakend_prometheus_service",
      "metric_reporting_period": 5,
      "trace_sample_rate": 1,
      "exporters": {
        "otlp": [
          {
            "disable_metrics": false,
            "disable_traces": true,
            "host": "localhost",
            "name": "five_seconds",
            "port": 5005,
            "use_http": true
          },
          {
            "disable_metrics": false,
            "disable_traces": true,
            "host": "localhost",
            "name": "eleven_seconds",
            "port": 5011,
            "use_http": true,
            "custom_metric_reporting_period": 11,
            "custom_reporting_period": 11
          }
        ]
      },
      "layers": {
        "global": {
          "disable_metrics": false,
          "disable_propagation": false,
          "disable_traces": false,
          "report_headers": true
        },
        "proxy": {
          "disable_metrics": false,
          "disable_traces": true,
          "report_headers": true
        },
        "backend": {
          "metrics": {
            "detailed_connection": true,
            "disable_stage": false,
            "read_payload": false,
            "round_trip": false
          },
          "traces": {
            "detailed_connection": false,
            "disable_stage": false,
            "read_payload": false,
            "report_headers": false,
            "round_trip": false
          }
        }
      }
    }
  },
  "endpoints": [
    {
      "endpoint": "/dashboard",
      "backend": [
        {
          "url_pattern": "/customer.json"
        },
        {
          "url_pattern": "/sales.json"
        }
      ]
    }
  ]
}

2025/03/03 09:58:56 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:01 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:02 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5011/v1/metrics": dial tcp 127.0.0.1:5011: connect: connection refused
2025/03/03 09:59:06 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:11 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:13 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5011/v1/metrics": dial tcp 127.0.0.1:5011: connect: connection refused
2025/03/03 09:59:16 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:21 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:24 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5011/v1/metrics": dial tcp 127.0.0.1:5011: connect: connection refused
2025/03/03 09:59:26 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:31 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused
2025/03/03 09:59:35 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5011/v1/metrics": dial tcp 127.0.0.1:5011: connect: connection refused
2025/03/03 09:59:36 KRAKEND ERROR: [SERVICE: OpenTelemetry] failed to upload metrics: Post "https://localhost:5005/v1/metrics": dial tcp 127.0.0.1:5005: connect: connection refused