prometheus-net

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using Grpc.AspNetCore.Server;
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using Microsoft.AspNetCore.Http;
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namespace Prometheus;
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// Modeled after HttpRequestMiddlewareBase, just with gRPC specific functionality.
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internal abstract class GrpcRequestMiddlewareBase<TCollector, TChild>
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    where TCollector : class, ICollector<TChild>
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    where TChild : class, ICollectorChild
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{
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    /// <summary>
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    /// The set of labels from among the defaults that this metric supports.
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    /// 
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    /// This set will be automatically extended with labels for additional
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    /// route parameters when creating the default metric instance.
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    /// </summary>
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    protected abstract string[] DefaultLabels { get; }
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    /// <summary>
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    /// Creates the default metric instance with the specified set of labels.
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    /// Only used if the caller does not provide a custom metric instance in the options.
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    /// </summary>
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    protected abstract TCollector CreateMetricInstance(string[] labelNames);
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    /// <summary>
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    /// The factory to use for creating the default metric for this middleware.
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    /// Not used if a custom metric is alreaedy provided in options.
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    /// </summary>
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    protected MetricFactory MetricFactory { get; }
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    private readonly TCollector _metric;
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    protected GrpcRequestMiddlewareBase(GrpcMetricsOptionsBase? options, TCollector? customMetric)
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    {
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        MetricFactory = Metrics.WithCustomRegistry(options?.Registry ?? Metrics.DefaultRegistry);
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        if (customMetric != null)
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        {
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            _metric = customMetric;
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            ValidateNoUnexpectedLabelNames();
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        }
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        else
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        {
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            _metric = CreateMetricInstance(DefaultLabels);
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        }
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    }
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    protected TChild? CreateChild(HttpContext context)
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    {
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        var metadata = context.GetEndpoint()?.Metadata?.GetMetadata<GrpcMethodMetadata>();
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        if (metadata == null)
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        {
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            // Not a gRPC request
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            return null;
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        }
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        if (!_metric.LabelNames.Any())
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        {
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            return _metric.Unlabelled;
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        }
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        return CreateChild(context, metadata);
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    }
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    protected TChild CreateChild(HttpContext context, GrpcMethodMetadata metadata)
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    {
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        var labelValues = new string[_metric.LabelNames.Length];
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        for (var i = 0; i < labelValues.Length; i++)
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        {
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            switch (_metric.LabelNames[i])
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            {
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                case GrpcRequestLabelNames.Service:
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                    labelValues[i] = metadata.Method.ServiceName;
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                    break;
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                case GrpcRequestLabelNames.Method:
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                    labelValues[i] = metadata.Method.Name;
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                    break;
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                default:
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                    // Should never reach this point because we validate in ctor.
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                    throw new NotSupportedException($"Unexpected label name on {_metric.Name}: {_metric.LabelNames[i]}");
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            }
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        }
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        return _metric.WithLabels(labelValues);
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    }
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    /// <summary>
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    /// If we use a custom metric, it should not have labels that are neither defaults nor additional route parameters.
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    /// </summary>
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    private void ValidateNoUnexpectedLabelNames()
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    {
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        var unexpected = _metric.LabelNames.Except(DefaultLabels);
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        if (unexpected.Any())
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            throw new ArgumentException($"Provided custom gRPC request metric instance for {GetType().Name} has some unexpected labels: {string.Join(", ", unexpected)}.");
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    }
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}