Allow configurable connection pool size

[angadsingh]

Proposal:
This client should allow configuring the underlying urllib3 connection pool size just like the older client:
influxdata/influxdb-python#349

Current behavior:
Currently this client throws the following error if there are more than 20 connections made:
WARNING:urllib3.connectionpool:Connection pool is full, discarding connection: eu-central-1-1.aws.cloud2.influxdata.com

Desired behavior:
Connection pool size should be configurable. E.g. in the old influx-python client there is a pool_size argument in the client's constructor:
https://influxdb-python.readthedocs.io/en/latest/api-documentation.html?highlight=pool#influxdb.InfluxDBClient

Use case:
This is necessary to avoid the above error and allow multi threaded clients to hit influx db 2.0 cloud properly

[angadsingh]

here's the hack i am currently using:

auth_header_name = "Authorization"
auth_header_value = "Token " + INFLUX_TOKEN
influx_client = InfluxDBClient(url="https://eu-central-1-1.aws.cloud2.influxdata.com", token=INFLUX_TOKEN)
conf = influx_client.api_client.configuration
conf.connection_pool_maxsize = 50
influx_client.api_client = ApiClient(configuration=conf,
                                     header_name=auth_header_name,
                                     header_value=auth_header_value)
return influx_client.write_api(write_options=SYNCHRONOUS)

the default connection_pool_maxsize is multiprocessing.cpu_count() * 5, which in my case on a raspberry pi 4 evaluates to 20. Not enough when one is making 50+ threads to send data to influx. Use case: a ping tool which pings 50 hosts parallely and sends results to influx (so its a very valid use case).

[bednar]

@angadsingh, thanks for using our client, we will take a look.