Currently the `index.look_ahead_time` index setting controls the `index.time_series_end_time` index setting when rolling over a tsdb data stream. The `index.look_ahead_time` setting default to 2 hours, which means the by default the `index.time_series_end_time` index setting of the new backing index will be 2 hours later than the `index.time_series.start_time` index setting. The the `index.time_series.start_time` and `index.time_series_end_time` index settings  control what `@timestamp` are allowed to be indexed.

Additionally the `index.look_ahead_time` setting is used to generate the `index.time_series.start_time` and `index.time_series_end_time` index settings when creating a new data stream for the first backing index. Based on the defaults of `index.look_ahead_time` setting, the  `index.time_series.start_time` index is set to current time - 2 hours and the `index.time_series_end_time` index setting is set to current time + 2 hours.

However for indexing the initial data setting the `index.time_series.start_time` index setting to 2 hours in the past sometimes creates too small time window. For example because data gets indexed starting from midnight that day. 

The idea is to add a `index.look_back_time` setting that controls how to generate the `index.time_series.start_time` index setting when creating the data stream (for the first backing index). This allows for creating a first backing index that accepts data that is earlier then current time - $index.look_ahead_time, without affecting how `index.time_series.start_time` and `index.time_series_end_time` index settings are generated during rollover.

I think a good default for the `index.look_back_time` setting would be 24 hours. Meaning that by default the `index.time_series.start_time` index setting for the first backing index of a new data stream would be current time - 24 hours.

Relates to elastic/integrations#7345
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
} catch (Exception | AssertionError e) {
            logger.error("search response body causing assertion error [" + responseBody + "]", e);
            throw e;
        }
    }
    public void testSimulateTsdbDataStreamTemplate() throws Exception {
        var simulateIndexTemplateRequest = new Request("POST", "/_index_template/_simulate_index/k8s");
        var response = client().performRequest(simulateIndexTemplateRequest);
        assertOK(response);
        var responseBody = entityAsMap(response);
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index"), aMapWithSize(6));
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index.number_of_shards"), equalTo("2"));
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index.number_of_replicas"), equalTo("0"));
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index.mode"), equalTo("time_series"));
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index.time_series.start_time"), notNullValue());
        assertThat(ObjectPath.evaluate(responseBody, "template.settings.index.time_series.end_time"), notNullValue());
        assertThat(
            ObjectPath.evaluate(responseBody, "template.settings.index.routing_path"),
            containsInAnyOrder("metricset", "k8s.pod.uid", "pod.labels.*")
        );
        assertThat(ObjectPath.evaluate(responseBody, "overlapping"), empty());
    }
    public void testSubsequentRollovers() throws Exception {
        var createDataStreamRequest = new Request("PUT", "/_data_stream/k8s");
        assertOK(client().performRequest(createDataStreamRequest));
        int numRollovers = 16;
        for (int i = 0; i < numRollovers; i++) {
            var rolloverRequest = new Request("POST", "/k8s/_rollover?pretty");
            var rolloverResponse = client().performRequest(rolloverRequest);
            assertOK(rolloverResponse);