s => s
.Size(0)
.Aggregations(a => a
    .DateHistogram("projects_started_per_month", dh => dh
        .Field(p => p.StartedOn)
        .Interval(DateInterval.Month)
        .Aggregations(aa => aa
            .Sum("commits", sm => sm
                .Field(p => p.NumberOfCommits)
            )
            .Filter("stable_state", f => f
                .Filter(ff => ff
                    .Term(p => p.State, "Stable")
                )
                .Aggregations(aaa => aaa
                    .Sum("commits", sm => sm
                        .Field(p => p.NumberOfCommits)
                    )
                )
            )
            .BucketScript("stable_percentage", bs => bs
                .BucketsPath(bp => bp
                    .Add("totalCommits", "commits")
                    .Add("stableCommits", "stable_state>commits")
                )
                .Script(ss =>ss.Inline("stableCommits / totalCommits * 100").Lang("groovy"))
            )
        )
    )
)

new SearchRequest<Project>()
{
    Size = 0,
    Aggregations = new DateHistogramAggregation("projects_started_per_month")
    {
        Field = "startedOn",
        Interval = DateInterval.Month,
        Aggregations =
            new SumAggregation("commits", "numberOfCommits") &&
            new FilterAggregation("stable_state")
            {
                Filter = new TermQuery
                {
                    Field = "state",
                    Value = "Stable"
                },
                Aggregations = new SumAggregation("commits", "numberOfCommits")
            } &&
            new BucketScriptAggregation("stable_percentage", new MultiBucketsPath
                {
                    { "totalCommits", "commits" },
                    { "stableCommits", "stable_state>commits" }
                })
            {
                Script = new InlineScript("stableCommits / totalCommits * 100") { Lang = "groovy" }
            }
    }
}

{
  "size": 0,
  "aggs": {
    "projects_started_per_month": {
      "date_histogram": {
        "field": "startedOn",
        "interval": "month"
      },
      "aggs": {
        "commits": {
          "sum": {
            "field": "numberOfCommits"
          }
        },
        "stable_state": {
          "filter": {
            "term": {
              "state": {
                "value": "Stable"
              }
            }
          },
          "aggs": {
            "commits": {
              "sum": {
                "field": "numberOfCommits"
              }
            }
          }
        },
        "stable_percentage": {
          "bucket_script": {
            "buckets_path": {
              "totalCommits": "commits",
              "stableCommits": "stable_state>commits"
            },
            "script": {
              "inline": "stableCommits / totalCommits * 100",
              "lang": "groovy"
            }
          }
        }
      }
    }
  }
}

response.ShouldBeValid();
var projectsPerMonth = response.Aggs.DateHistogram("projects_started_per_month");
projectsPerMonth.Should().NotBeNull();
projectsPerMonth.Buckets.Should().NotBeNull();
projectsPerMonth.Buckets.Count.Should().BeGreaterThan(0);

foreach(var item in projectsPerMonth.Buckets)
{
    var stablePercentage = item.BucketScript("stable_percentage");
    stablePercentage.Should().NotBeNull();
    stablePercentage.Value.Should().HaveValue();
}