feat: WithGRPCDialOption for custom gRPC dial settings

[mlwelles]

What this adds

WithGRPCDialOption(opt grpc.DialOption) — a general escape hatch for gRPC dial
settings the dedicated options do not cover: TLS transport credentials,
interceptors, keepalive parameters, and so on. Applied when opening a remote
(dgraph://) connection; ignored for embedded (file://) URIs.

client, err := mg.NewClient(
    "dgraph://localhost:9080",
    mg.WithGRPCDialOption(grpc.WithTransportCredentials(creds)),
    mg.WithGRPCDialOption(grpc.WithKeepaliveParams(kp)),
)

The problem it solves

The client exposed only a fixed set of gRPC knobs (e.g. WithMaxRecvMsgSize).
Anything else — mTLS, auth interceptors, custom keepalive — required forking the
connection code. WithGRPCDialOption lets callers pass any grpc.DialOption
straight through to the dial, so the client covers the long tail of gRPC
configuration without growing a dedicated option for each.

WithMaxRecvMsgSize is preserved and folded into the same dial-option assembly,
so the two compose rather than conflict.

Review round — changes since first push

Addresses the automated review (cubic):

• Dedup cache key was lossy (P1): the client dedup key counted dial options
  (len(...)), so two clients to the same URI with one different option each
  produced the same key and were incorrectly merged. grpc.DialOption values
  are opaque and not comparable, so the key now includes each option's runtime
  identity. Differently configured clients are never merged; the cache errs
  toward keeping them apart.
• Embedded consistency: dial options now contribute to the key only for
  remote (dgraph://) URIs, matching the documented "ignored for file://"
  behavior.
• Weak test (P1): the cache-key test only checked 0-vs-1 option count. It
  now asserts that two clients with the same count but different options get
  different keys.

Tests

• TestKeyDistinguishesGRPCDialOptions — different options at the same count
  yield different keys.
• TestKeyIgnoresGRPCDialOptionsForEmbedded — dial options do not affect the
  key for file:// clients.
• TestWithGRPCDialOptionAppends — options accumulate.

Documentation

A runnable ExampleWithGRPCDialOption showing TLS credentials and keepalive.

[cubic-dev-ai]

2 issues found across 2 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="dial_options_test.go">

<violation number="1" location="dial_options_test.go:27">
P1: TestKeyDistinguishesGRPCDialOptions only validates 0 vs 1 dial option count, not different values, masking a cache-key collision bug.</violation>
</file>

<file name="client.go">

<violation number="1" location="client.go:456">
P1: Cache key disambiguation for custom gRPC dial options is lossy (count-only), allowing collisions for different options and inconsistent behavior with documented embedded-mode ignore semantics.</violation>
</file>

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