Add Transport config parameter for MaxConnsPerHost

[ccapurso]

Rendering templates within Vault Agent that contain many secrets can result in aggressively opening a very large number of TCP connections all at once. Opening this many connections essentially results in a DoS against Vault. This PR adds the ability to specify a value for MaxConnsPerHost within a client's Transport configuration. Support for this new field was only added for the Vault client.

Fixes #1840

[lornasong]

Hey @ccapurso! These changes make sense to me.

I'm not super familiar with this code so did a review by comparing how a similar field(I chose MaxIdleConnsPerHost) is implemented. I noticed a couple additional changes with MaxIdleConnsPerHost:

• transport.go GoString() - it doesn't look like all transport fields are in the GoString. I'm not sure if it would be worthwhile to add MaxConnsPerHost here?
• config_test.go TestParse - it looks like adding a test case for MaxConnsPerHost might keep parity?
• cli.go ParseFlags() - would it make sense to add flag support for this field?

Totally okay ifMaxConnsPerHost doesn't make sense to be in the 3 places above, so just want to get your input.

Another question mostly because I'm not familiar with things, would you mind sharing a little bit about how you tested these changes? Appreciate it, thank you

[ccapurso]

@lornasong, thank you for the review! I have addressed the three spots you mentioned:

• transport.go GoString()
• config_test.go TestParse - - Only added support for Vault at this time
• cli.go ParseFlags() - Only added support for Vault at this time

I tested using the reproduction steps provided in #1840 and ensured that the connections were limited based on the configured maximum.

[lornasong]

thanks @ccapurso! the changes make sense to me