Redesign allocations for line protocol

[fntlnz]

I'm very unsure about the usage of the buffer here:

nginx-influxdb-module/src/ngx_http_influxdb_metric.c

Lines 87 to 96 in 3ecba97

	(void)ngx_sprintf(buf->last,
	"%V,server_name=%V "
	"method=\"%V\",status=%i,bytes_sent=%O,body_"
	"bytes_sent=%O,header_"
	"bytes_sent=%z,request_length=%O,uri=\"%V\",extension=\"%"
	"V\",content_type=\"%V\",request_time=%V",
	&measurement, &m->server_name, &m->method, m->status,
	m->bytes_sent, m->body_bytes_sent, m->header_bytes_sent,
	m->request_length, &m->uri, &m->extension, &m->content_type,
	&m->request_time);

I have the feeling that it doesn't handle very well its boundaries, opening this to convert that to a mix of ngx_palloc and ngx_memcpy that then are used to create an ngx_str_t