You can iterate through all points using TrailRenderer.GetPositions like e.g.

public static class TrailRendererExtensions
{
    public static float GetTrailLength(this TrailRenderer trailRenderer)
    {
        // You store the count since the created array might be bigger than the actual pointCount

        // Note: apparently the API is wrong?! There it says starting from version 2018.1 the parameter is "out"
        // if you get an error for the "out" anyway or if you use older versions instead use
        //var points = new Vector3[trailRenderer.positionCount]; 
        //var count = trailRenderer.GetPositions(points);
        var count = trailRenderer.GetPositions(out var points);
    
        // If there are not at least 2 points .. well there is nothing to measure
        if(count < 2) return 0f;
    
        var length = 0f;
    
        // Store the first position 
        var start = points[0];

        // Iterate through the rest of positions
        for(var i = 1; i < count; i++)
        {
            // get the current position
            var end = points[i];
            // Add the distance to the last position
            // basically the same as writing
            //length += (end - start).magnitude;
            length += Vector3.Distance(start, end);
            // update the start position for the next iteration
            start = end;
        }
        return length;
    }
}

this is now an extension method so you can simply use it anywhere in your project like

var length = yourTrailRenderer.GetTrailLength();