Renameable time field

[badge]

Description

This PR enables two new features in the crate:

1. Structs with DateTime<Utc> fields that become nanosecond-encoded uint64s
2. Setting a DateTime<Utc> field other than time to be the time field for the struct in the InfluxDbWriteable proc macro

For (1), this is now possible:

    #[derive(Debug, Deserialize, InfluxDbWriteable)]
    struct WeatherReading {
        time: DateTime<Utc>,
        humidity: i32,
        #[influxdb(tag)]
        wind_direction: String,
        forecast_time: DateTime<Utc>,
    }

Previously, forecast_time would have not worked. As type have to implement From<T> rather than TryFrom<T>, it defaults to zero if the DateTime<Utc> field is OOB.

For (2), we can now do:

    #[derive(Debug, Deserialize, InfluxDbWriteable)]
    struct WeatherReading {
        #[influxdb(time)]
        #[serde(rename = "time")]
        reading_time: DateTime<Utc>,
        humidity: i32,
        #[influxdb(tag)]
        wind_direction: String,
        forecast_time: DateTime<Utc>,
    }

The derive macro does sensible checks:

1. It enumerates the struct's fields and checks for any tagged with #[influxdb(time)] or named time and not tagged with #[influxdb(ignore)]
2. If it finds more than one such field, it gives a compile error
3. If it finds no such fields, it gives a compile error
4. Otherwise, it uses the name of the field as the time of the created WriteQuery instance

Have to dash but opening anyway, will be back soon!

[msrd0]

Thanks for your PR!

[Empty2k12]

LGTM 🚀 Does it make sense to amend the ticket you created about the 0 problematic, or open a separate one?

[msrd0]

Yeah sure we can track that under the same ticket