Change default for "enable_http2" in prometheus.remote_write

[ptodev]

Request

As reported by @bboreham, setting enable_http2 to true causes all shards to be sent over one socket. This is not performant at high data rates. We should consider changing the default value to false.

It is not clear to me whether this might break something for users who rely on enable_http2 being true. We need to look into it before changing the defaults.

Use case

Better performance.