feat(core): add Prometheus exposition format parser

src/Vantage.Core/Prometheus/PrometheusSample.cs

@@ -0,0 +1,13 @@
+namespace Vantage.Core.Prometheus;
+
+public sealed record PrometheusSample(
+    string Name,
+    IReadOnlyDictionary<string, string> Labels,
+    double Value,
+    long? TimestampMs)
+{
+    public string? Label(string key) => Labels.TryGetValue(key, out var value) ? value : null;
+
+    public bool HasLabel(string key, string expected) =>
+        Labels.TryGetValue(key, out var value) && value == expected;
+}

src/Vantage.Core/Prometheus/PrometheusTextParser.cs

@@ -0,0 +1,169 @@
+using System.Globalization;
+using System.Text;
+
+namespace Vantage.Core.Prometheus;
+
+public static class PrometheusTextParser
+{
+    private static readonly IReadOnlyDictionary<string, string> NoLabels =
+        new Dictionary<string, string>();
+
+    public static IReadOnlyList<PrometheusSample> Parse(string exposition)
+    {
+        var samples = new List<PrometheusSample>();
+
+        foreach (var rawLine in exposition.Replace("\r\n", "\n").Split('\n'))
+        {
+            var line = rawLine.Trim();
+            if (line.Length == 0 || line[0] == '#')
+                continue;
+
+            var sample = ParseLine(line);
+            if (sample is not null)
+                samples.Add(sample);
+        }
+
+        return samples;
+    }
+
+    private static PrometheusSample? ParseLine(string line)
+    {
+        var cursor = 0;
+        var name = ReadName(line, ref cursor);
+        if (name.Length == 0)
+            return null;
+
+        var labels = NoLabels;
+        SkipSpaces(line, ref cursor);
+        if (cursor < line.Length && line[cursor] == '{')
+        {
+            labels = ReadLabels(line, ref cursor);
+            SkipSpaces(line, ref cursor);
+        }
+
+        var remainder = line[cursor..].Trim();
+        if (remainder.Length == 0)
+            return null;
+
+        var parts = remainder.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        if (!TryParseValue(parts[0], out var value))
+            return null;
+
+        long? timestamp = null;
+        if (parts.Length >= 2 && long.TryParse(parts[1], NumberStyles.Integer, CultureInfo.InvariantCulture, out var ts))
+            timestamp = ts;
+
+        return new PrometheusSample(name, labels, value, timestamp);
+    }
+
+    private static string ReadName(string line, ref int cursor)
+    {
+        var start = cursor;
+        while (cursor < line.Length)
+        {
+            var c = line[cursor];
+            if (c == '{' || c == ' ' || c == '\t')
+                break;
+            cursor++;
+        }
+
+        return line[start..cursor];
+    }
+
+    private static IReadOnlyDictionary<string, string> ReadLabels(string line, ref int cursor)
+    {
+        var labels = new Dictionary<string, string>(StringComparer.Ordinal);
+        cursor++;
+
+        while (cursor < line.Length && line[cursor] != '}')
+        {
+            SkipSpaces(line, ref cursor);
+            var keyStart = cursor;
+            while (cursor < line.Length && line[cursor] != '=' && line[cursor] != '}')
+                cursor++;
+
+            if (cursor >= line.Length || line[cursor] != '=')
+                break;
+
+            var key = line[keyStart..cursor].Trim();
+            cursor++;
+            SkipSpaces(line, ref cursor);
+
+            if (cursor >= line.Length || line[cursor] != '"')
+                break;
+
+            var value = ReadQuoted(line, ref cursor);
+            if (key.Length > 0)
+                labels[key] = value;
+
+            SkipSpaces(line, ref cursor);
+            if (cursor < line.Length && line[cursor] == ',')
+                cursor++;
+        }
+
+        if (cursor < line.Length && line[cursor] == '}')
+            cursor++;
+
+        return labels;
+    }
+
+    private static string ReadQuoted(string line, ref int cursor)
+    {
+        var builder = new StringBuilder();
+        cursor++;
+
+        while (cursor < line.Length)
+        {
+            var c = line[cursor];
+            if (c == '\\' && cursor + 1 < line.Length)
+            {
+                var next = line[cursor + 1];
+                builder.Append(next switch
+                {
+                    'n' => '\n',
+                    't' => '\t',
+                    '"' => '"',
+                    '\\' => '\\',
+                    _ => next
+                });
+                cursor += 2;
+                continue;
+            }
+
+            if (c == '"')
+            {
+                cursor++;
+                break;
+            }
+
+            builder.Append(c);
+            cursor++;
+        }
+
+        return builder.ToString();
+    }
+
+    private static bool TryParseValue(string token, out double value)
+    {
+        switch (token)
+        {
+            case "+Inf":
+                value = double.PositiveInfinity;
+                return true;
+            case "-Inf":
+                value = double.NegativeInfinity;
+                return true;
+            case "NaN":
+                value = double.NaN;
+                return true;
+            default:
+                return double.TryParse(token, NumberStyles.Float, CultureInfo.InvariantCulture, out value);
+        }
+    }
+
+    private static void SkipSpaces(string line, ref int cursor)
+    {
+        while (cursor < line.Length && (line[cursor] == ' ' || line[cursor] == '\t'))
+            cursor++;
+    }
+}